Towards an accurate molecular orbital theory for excited states : Ethene, butadiene, and hexatriene

A newly proposed quantum chemical approach for ab initio calculations of electronic spectra of molecular systems is applied to the molecules ethene, trans‐1,3‐butadiene, and trans‐trans‐1,3,5‐hexatriene. The method has the aim of being accurate to better than 0.5 eV for excitation energies and is expected to provide structural and physical data for the excited states with good reliability. The approach is based on the complete active space (CAS) SCF method, which gives a proper description of the major features in the electronic structure of the excited state, independent of its complexity, accounts for all near degeneracy effects, and includes full orbital relaxation. Remaining dynamic ele…

Computation of conical intersections by using perturbation techniques

Multiconfigurational second-order perturbation theory, both in its single-state multiconfigurational second-order perturbation theory (CASPT2) and multistate (MS-CASPT2) formulations, is used to search for minima on the crossing seams between different potential energy hypersurfaces of electronic states in several molecular systems. The performance of the procedures is tested and discussed, focusing on the problem of the nonorthogonality of the single-state perturbative solutions. In different cases the obtained structures and energy differences are compared with available complete active space self-consistent field and multireference configuration interaction solutions. Calculations on dif…

Tuning the photophysical properties of anti-B18H22: efficient intersystem crossing between excited singlet and triplet states in new 4,4'-(HS)2-anti-B18H20.

The tuning of the photophysical properties of the highly fluorescent boron hydride cluster anti-B18H22 (1), by straightforward chemical substitution to produce 4,4'-(HS)2-anti-B18H20 (2), facilitates intersystem crossing from excited singlet states to a triplet manifold. This subsequently enhances O2((1)Δg) singlet oxygen production from a quantum yield of ΦΔ ∼ 0.008 in 1 to 0.59 in 2. This paper describes the synthesis and full structural characterization of the new compound 4,4'-(HS)2-anti-B18H20 (2) and uses UV-vis spectroscopy coupled with density functional theory (DFT) and ab initio computational studies to delineate and explain its photophysical properties.

Cyclobutane Pyrimidine Photodimerization of DNA/RNA Nucleobases in the Triplet State

The photoinduced formation of cyclobutane pyrimidine dimers in the triplet excited state of the DNA/RNA pyrimidine nucleobases pairs has been studied at the CASPT2 level of theory. A stepwise mechanism through the triplet state of the homodimer is proposed for the pairs of nucleobases cytosine, thymine, and uracil involving a singlet−triplet crossing intermediary structure of biradical character representing the most favorable triplet state conformation of the nucleobases as found in the DNA environment. The efficiency of the mechanism will be modulated by two factors: the effectiveness of the triplet−triplet energy transfer process from a donor photosensitizer molecule, which relates to th…

Communication: Electronic UV-Vis transient spectra of the ·OH reaction products of uracil, thymine, cytosine, and 5,6-dihydrouracil by using the complete active space self-consistent field second-order perturbation (CASPT2//CASSCF) theory

Addition of ∙OH radicals to pyrimidine nucleobases is a common reaction in DNA/RNA damage by reactive oxygen species. Among several experimental techniques, transient absorption spectroscopy has been during the last decades used to characterize such compounds. Discrepancies have however appeared in the assignment of the adduct or adducts responsible for the reported transient absorption UV-Vis spectra. In order to get an accurate assignment of the transient spectra and a unified description of the absorption properties of the ∙OH reaction products of pyrimidines, a systematic complete active space self-consistent field second-order perturbation (CASPT2//CASSCF) theory study has been carried…

The Role of Adenine Excimers in the Photophysics of Oligonucleotides

Energies and structures of different arrangements of the stacked adenine homodimer have been computed at the ab initio CASPT2 level of theory in isolation and in an aqueous environment. Adenine dimers are shown to form excimer singlet states with different degrees of stacking and interaction. A model for a 2-fold decay dynamics of adenine oligomers can be supported in which, after initial excitation in the middle UV range, unstacked or slightly stacked pairs of nucleobases will relax by an ultrafast internal conversion to the ground state, localizing the excitation in the monomer and through the corresponding conical intersection with the ground state. On the other hand, long-lifetime intra…

DNA nucleobase properties and photoreactivity: Modeling environmental effects

Abstract The accurate ab initio quantum chemical (QM) method multiconfigurational second-order perturbation (CASSPT2)/complete active space self-consistent field (CASSCF) has been used in conjunction with molecular mechanics (MM) procedures to compute molecular properties and photoinduced reactivity of DNA/RNA nucleobases (NABs) in isolation and within a realistic environment, in which the double helix strand, the aqueous media, and the external counterions are included. It is illustrated that the use of an MM model is helpful both to account for short- and long-range effects of the system surrounding the QM molecular core and to provide the proper structural constraints that allow more acc…

A theoretical determination of the dissociation energy of the nitric oxide dimer

Multi-reference CI methods have been applied to determine the dissociation energy and structure of thecis-N2O2 molecule. The convergence of the theoretical result has been checked with respect to a systematic expansion of the one-electron basis set and the multi-reference CI wave function. The best calculated value, 13.8 kJ/mol, is in agreement with the experimental value, 12.2 kJ/mol. It has been obtained with an extended ANO-type basis set [6s5p3d2f], including the effect of the basis set superposition error (BSSE) in the geometry optimization, and additional effects, such as the electron correlation of core electrons and relativistic corrections, using the average coupled pair functional…

A Theoretical Insight into the Photophysics of Acridine

The electronic absorption and emission spectra of acridine have been studied by means of a multiconfigurational second-order perturbation method (CASSCF/CASPT2) and its multistate extension (MS-CASPT2). The low-lying valence singlet and triplet π → π* and n → π* excited states have been computed. The location of the lowest Rydberg state (3s) has been also estimated. By optimization of the geometries of the ground and low-lying excited states and the calculation of transition energies and properties, the obtained results lead to a complete analysis and assignment of the available experimental singlet−singlet and triplet−triplet absorption spectra and to the description of the basic features …

Relaxation Mechanisms of 5-Azacytosine.

The photophysics and deactivation pathways of the noncanonical 5-azacytosine nucleobase were studied using the CASPT2//CASSCF protocol. One of the most significant differences with respect to the parent molecule cytosine is the presence of a dark (1)(nNπ*) excited state placed energetically below the bright excited state (1)(ππ*) at the Franck-Condon region. The main photoresponse of the system is a presumably efficient radiationless decay back to the original ground state, mediated by two accessible conical intersections involving a population transfer from the (1)(ππ*) and the (1)(nNπ*) states to the ground state. Therefore, a minor contribution of the triplet states in the photophysics o…

Theoretical CASPT2 study of the excited state double proton transfer reaction in the 7-azaindole dimer

Accurate CASPT2 calculations on the excited state double proton transfer reaction in the 7-azaindole dimer have been performed on different symmetric and asymmetric pathways along the protons interchange. The presence of a conical intersection connecting the initially photoinduced singlet excited state with a charge transfer state leading both to asymmetric ionic and neutral structures promotes a step-wise mechanism, probably taking place through the ionic intermediate. The concerted path is computed slightly higher in energy in the gas phase.

Theoretical Study of the Electronic Spectrum of trans-Stilbene

The electronic spectrum of trans-stilbene in the energy range up to 6 eV has been studied using multiconfigurational second-order perturbation theory (CASPT2). The study includes a geometry determination of the ground state. In all, 12 singlet and one triplet excited states were studied. The calculated spectrum makes it possible to assign the valence excited singlet states corresponding to the three bands observed in the low-energy region of the one-photon absorption spectrum. The most intense feature of the calculated spectrum corresponds to the 11Ag → 21Bu transition at 4.07 eV. The weakly allowed 11Bu state was found 0.3 eV below 21Bu. Transition to the 31Ag state, computed at 4.95 eV, i…

A theory ofnonverticaltriplet energy transfer in terms of accurate potential energy surfaces: The transfer reaction from π,π* triplet donors to 1,3,5,7-cyclooctatetraene

Triplet energy transfer (TET) from aromatic donors to 1,3,5,7-cyclooctatetraene (COT) is an extreme case of "nonvertical" behavior, where the transfer rate for low-energy donors is considerably faster than that predicted for a thermally activated (Arrhenius) process. To explain the anomalous TET of COT and other molecules, a new theoretical model based on transition state theory for nonadiabatic processes is proposed here, which makes use of the adiabatic potential energy surfaces (PES) of reactants and products, as computed from high-level quantum mechanical methods, and a nonadiabatic transfer rate constant. It is shown that the rate of transfer depends on a geometrical distortion paramet…

Ultrafast internal conversion of excited cytosine via the lowest pipi electronic singlet state.

Computational evidence at the CASPT2 level supports that the lowest excited state pipi* contributes to the S1/S0 crossing responsible for the ultrafast decay of singlet excited cytosine. The computed radiative lifetime, 33 ns, is consistent with the experimentally derived value, 40 ns. The nOpi* state does not play a direct role in the rapid repopulation of the ground state; it is involved in a S2/S1 crossing. Alternative mechanisms through excited states pisigma* or nNpi* are not competitive in cytosine.

Ab initio determination of the ionization potentials of DNA and RNA nucleobases

Quantum chemical high level ab initio coupled-cluster and multiconfigurational perturbation methods have been used to compute vertical and adiabatic ionization potentials of the five canonical DNA and RNA nucleobases: uracil, thymine, cytosine, adenine, and guanine. Several states of their cations have been also calculated. The present results represent a systematic compendium of these magnitudes, establishing theoretical reference values at a level not reported before, calibrating computational strategies, and guiding the assignment of the features in the experimental photoelectron spectra. Daniel.Roca@uv.es Mercedes.Rubio@uv.es Manuela.Merchan@uv.es Luis.Serrano@uv.es

Theoretical Study of the Hydroxyl Radical Addition to Uracil and Photochemistry of the Formed U6OH• Adduct

Hydroxyl radical ((•)OH) is produced in biological systems by external or endogenous agents. It can damage DNA/RNA by attacking pyrimidine nucleobases through the addition to the C5═C6 double bond. The adduct resulting from the attachment at the C5 position prevails in the experimental measurements, although the reasons for this preference remain unclear. The first aim of this work is therefore to shed light on the comprehension of this important process. Thus, the thermal (•)OH addition to the C5═C6 double bond of uracil has been studied theoretically by using DFT, MP2, and the multiconfigurational CASPT2//CASSCF methodologies. The in-vacuo results obtained with the latter protocol plus th…

Structure, Spectroscopy, and Spectral Tuning of the Gas-Phase Retinal Chromophore: The β-Ionone "Handle" and Alkyl Group Effect

The low-lying singlet states (i.e. S0, S1, and S2) of the chromophore of rhodopsin, the protonated Schiff base of 11-cis-retinal (PSB11), and of its all-trans photoproduct have been studied in isolated conditions by using ab initio multiconfigurational second-order perturbation theory. The computed spectroscopic features include the vertical excitation, the band origin, and the fluorescence maximum of both isomers. On the basis of the S0-->S1 vertical excitation, the gas-phase absorption maximum of PSB11 is predicted to be 545 nm (2.28 eV). Thus, the predicted absorption maximum appears to be closer to that of the rhodopsin pigment (2.48 eV) and considerably red-shifted with respect to that…

On the role of the triplet state in the cis/trans isomerization of rhodopsin: A CASPT2//CASSCF study of a model chromophore

The possibility of population of the lowest-lying triplet state (T1) in the early events of the photochemical isomerization process of a model chromophore of Rhodopsin (Rh) has been analyzed using multireference perturbation theory (CASPT2//CASSCF) methods. It is shown that the characteristics of the isomerization process namely small S1−T1 gap, presence of hydrogen out of plane active vibrational modes, and existence of a dense manifold of vibrational states, render possible the fulfilment of the conditions needed for the population of T1. The possible consequences for the photochemistry and photophysics of Rh are also discussed. © 2011 Wiley Periodicals, Inc. Int J Quantum Chem 111:3431–3…

Theoretical insight into the intrinsic ultrafast formation of cyclobutane pyrimidine dimers in UV-irradiated DNA: thymine versus cytosine.

The higher formation yields measured in the ultrafast photoinduced formation of cyclobutane thymine dimers (T T) with respect to those of cytosine (C C) are explained, on the basis of ab initio CASPT2 results, by the existence in thymine of more reactive orientations and a less efficient photoreversibility, whereas in cytosine the funnel toward the photolesion becomes competitive with that mediating the internal conversion of the excited-cytosine monomer.

The chemical bonds in CuH, Cu2, NiH, and Ni2 studied with multiconfigurational second order perturbation theory

The performance of multiconfigurational second order perturbation theory has been analyzed for the description of the bonding in CuH, Cu2, NiH, and Ni2. Large basis sets based on atomic natural orbitals (ANOS) were employed. The effects of enlarging the active space and including the core‐valence correlation contributions have also been analyzed. Spectroscopic constants have been computed for the corresponding ground state. The Ni2 molecule has been found to have a 0+g ground state with a computed dissociation energy of 2.10 eV, exp. 2.09 eV, and a bond distance of 2.23 Å. The dipole moments of NiH and CuH are computed to be 2.34 (exp. 2.4±0.1) and 2.66 D, respectively. pou@uv.es ; merchan@…

On the absorbance changes in the photocycle of the photoactive yellow protein: A quantum-chemical analysis

Spectral changes in the photocycle of the photoactive yellow protein (PYP) are investigated by using ab initio multiconfigurational second-order perturbation theory at the available structures experimentally determined. Using the dark ground-state crystal structure [Genick, U. K., Soltis, S. M., Kuhn, P., Canestrelli, I. L. & Getzoff, E. D. (1998) Nature (London) 392, 206–209], the ππ* transition to the lowest excited state is related to the typical blue-light absorption observed at 446 nm. The different nature of the second excited state ( n π*) is consistent with the alternative route detected at 395-nm excitation. The results suggest the low-temperature photoproduct PYP HL as the mo…

Molecular Basis of DNA Photodimerization: Intrinsic Production of Cyclobutane Cytosine Dimers

Based on CASPT2 results, the present contribution establishes for the first time that cytosine photodimer formation (CC) is mediated along the triplet and singlet manifold by a singlet-triplet crossing, (T1/S0)X, and by a conical intersection, (S1/S0)CI, respectively. The former can be accessed in a barrierless way from a great variety of photochemical avenues and exhibits a covalent single bond between the ethene C6-C6' carbon atoms of each monomer. The efficiency of the stepwise triplet mechanism, however, would be modulated by the effectiveness of the intersystem crossing mechanism. The results provide the grounds for the understanding of the potential photogenotoxicity of endogenous and…

Theoretical Insight into the Spectroscopy and Photochemistry of Isoalloxazine, the Flavin Core Ring

The electronic singlet-singlet and singlet-triplet electronic transitions of the isoalloxazine ring of the flavin core are studied using second-order perturbation theory within the framework of the CASPT2//CASSCF protocol. The main features of the absorption spectrum are computed at 3.09, 4.28, 4.69, 5.00, and 5.37 eV. The lowest singlet (S1) and triplet (T1) excited states are found to be both of pi character with a singlet-triplet splitting of 0.57 eV. On the basis of the analysis of the computed spin-orbit couplings and the potential energy hypersurfaces built for the relevant excited states, the intrinsic mechanism for photoinduced population of T1 is discussed. Upon light absorption, e…

Singlet oxygen generation in PUVA therapy studied using electronic structure calculations

Abstract The ability of furocoumarins to participate in the PUVA (Psoralen + UV-A) therapy against skin disorders and some types of cancer, is analyzed on quantum chemical grounds. The efficiency of the process relies on its capability to populate its lowest triplet excited state, and then either form adducts with thymine which interfere DNA replication or transfer its energy, generating singlet molecular oxygen damaging the cell membrane in photoactivated tissues. By determining the spin–orbit couplings, shown to be the key property, in the intersystem crossing yielding the triplet state of the furocoumarin, the electronic couplings in the triplet–triplet energy transfer process producing …

On the intrinsic population of the lowest triplet state of uracil

Abstract From CASPT2//CASSCF quantum-chemical computations it is determined that the lowest triplet state of uracil can be efficiently populated from the initially activated singlet manifold through respective singlet–triplet crossings of the singlet state with the low-lying 3nπ∗ state at 4.6 eV and with the lowest 3ππ∗ state at 4.2 eV located along the minimum energy path of the low-lying 1ππ∗ state. Large spin–orbit coupling elements predict, in particular for the former case, efficient intersystem crossing processes. The wavelength dependence measured for the triplet quantum yield can be explained by the location of the singlet–triplet crossing regions.

Theoretical characterization of the lowest-energy absorption band of pyrrole

The lowest-energy band of the electronic spectrum of pyrrole has been studied with vibrational resolution by using multiconfigurational second-order perturbation theory (CASPT2) and its multistate extension (MS–CASPT2) in conjunction with large atomic natural orbital-type basis sets including Rydberg functions. The obtained results provide a consistent picture of the recorded spectrum in the energy region 5.5–6.5 eV and confirm that the bulk of the intensity of the band arises from a ππ∗ intravalence transition, in contradiction to recent theoretical claims. Computed band origins for the 3s,3p Rydberg electronic transitions are in agreement with the available experimental data, although new…

A theoretical study of the collinear reaction F+H2→HF+H using multiconfigurational second-order perturbation theory (CASPT2)

Abstract The second-order perturbation method (CASPT2) with a single state multiconfigurational reference function generated in complete active self-consistent field (CASSCF) calculations has been used to compute the collinear barrier height, saddle point geometry, and exothermicity of the reaction F+H 2 →HF+H. Comparison with full configuration (FCI) calculations with small basis sets shows that the CASPT2 method is capable of reproducing accurately the exact benchmark results correlating seven electrons. Large atomic natural orbital basis sets are used at the seven- and nine-electron level of correlation. With the largest ANO basis set used, F[7s6p5d4f2g]/H[6s5p4d2f], the computed nine-el…

Theoretical Analysis of the Excited States in Maleimide

The electronic excited states of maleimide have been studied using multiconfigurational second-order perturbation theory in its multistate formulation (MS-CASPT2) and extended atomic natural orbita...

On the Deactivation Mechanisms of Adenine–Thymine Base Pair

In this contribution, the multiconfigurational second-order perturbation theory method based on a complete active space reference wave function (CASSCF/CASPT2) is applied to study all possible single and double proton/hydrogen transfers between the nucleobases in the adenine-thymine (AT) base pair, analyzing the role of excited states with different nature [localized (LE) and charge transfer (CT)], and considering concerted as well as step-wise mechanisms. According to the findings, once the lowest excited states, localized in adenine, are populated during UV irradiation of the Watson-Crick base pair, the proton transfer in the N-O bridge does not require high energy in order to populate a …

Determination of the lowest-energy oxidation site in nucleotides: 2'-deoxythymidine 5'-monophosphate anion.

High level ab initio computations anticipate nucleobases as the most favorable sites for oxidation in nucleotides. At the CASPT2 level, the lowest ionization channel for the 2'-deoxythymidine 5'-monophosphate anion is related to a pi-orbital of the thymine base. The present findings lead to revision of the recent assignments of the photodetachment photoelectron spectra of mononucleotide anions in the gas phase and support the classical view of the nucleobase being the main actor in the oxidation process of both nucleosides and nucleotides.

The role of pyrimidine nucleobase excimers in DNA photophysics and photoreactivity

Abstract Quantum chemical studies using the accurate CASPT2//CASSCF procedure show that π-stacked interactions in biochromophores such as pyrimidine (Pyr) DNA/RNA nucleobases pairs yield excimer-like situations which behave as precursors of processes like charge transfer (CT) or photoreactivity and are the source of the emissive properties in DNA. Examples are the CT between adjacent DNA nucleobases in a strand of oligonucleotides and the photodimerization taking place in cytosine (C) pairs leading to cyclobutanecytosine (CBC) mutants. These processes take place through nonadiabatic photochemical mechanisms whose evolution is determined by the presence and accessibility of conical intersect…

Toward an Understanding of Ultrafast Electron Transfer in Photosynthesis

The mechanism of electron transfer (ET) from reduced pheophytin (Pheo−) to the primary stable photosynthetic acceptor, a quinone (Q) molecule, is addressed by using high-level ab initio computations and realistic molecular models. The results reveal that the ET process involving the (Pheo−+Q) and (Pheo+Q−) oxidation states can be seen essentially as an ultrafast radiationless transition between the two hypersurfaces taking place via conical intersections (CIs) and is favoured when the topology of the interacting moieties makes possible some overlap between the lowest occupied molecular orbitals (LUMO) of the two systems. Thus, it is anticipated that large scale motions, which are difficult …

Mechanism of the OH Radical Addition to Adenine from Quantum-Chemistry Determinations of Reaction Paths and Spectroscopic Tracking of the Intermediates.

The OH radical is a well-known mediator in the oxidation of biological structures like DNA. Over the past decades, the precise events taking place after reaction of DNA nucleobases with OH radical have been widely investigated by the scientific community. Thirty years after the proposal of the main routes for the reaction of •OH with adenine (Vieira, A.; Steenken, S. J. Am. Chem. Soc. 1990, 112, 6986−6994), the present work demonstrates that the OH radical addition to C4 position is a minor pathway. Instead, the dehydration process is mediated by the A5OH adduct. Conclusions are based on density functional theory calculations for the ground-state reactivity and highly accurate multiconfigur…

Multiconfigurational perturbation theory (CASPT2) applied to the study of the low-lying singlet and triplet excited states of cyclopropene

The electronic spectrum of cyclopropene has been studied using multiconfigurational second-order perturbation theory (CASPT2) with extended ANO-type basis sets. The calculation comprises two valence states and the 3s, 3p, 3d members of the Rydberg series converging to the π and σ ionization limits. A total of twenty singlet and twenty triplet excited states have been analyzed. The results confirm the valence nature of the lowest energy singlet-singlet band and yield a conclusive assignment: the first dipole-allowed transition in cyclcopropene is due to absorption to a (σ → π*) state. The (π → π*) (V) state is interleaved among a number of Rydberg states in the most intense band of the syste…

An ab initio CI study on the rotational barrier of the allyl anion

All-electron and pseudopotential non-empirical calculations have been performed on C 2v and C s (syn, anti) allyl anion conformations. Using a double-zeta valence-shell basis set within the Epstein-Nesbet definition of the unperturbed Hamiltonian, a value about 19 kcal/mol is found for the barrier to rotation of the allyl anion. This value is the theoretical value obtained with greater accuracy, and the lowest one for the rotational barrier.

Photoreactivity of Furocoumarins and DNA in PUVA Therapy: Formation of Psoralen−Thymine Adducts

The mechanism of the [2 + 2] cycloaddition photoreaction of psoralen and a DNA nucleobase, thymine, cornerstone of the furocoumarin-based PUVA (psoralen + UVA radiation) phototherapy, has been studied by the quantum-chemical multiconfigurational CASPT2 method. Triplet- and singlet-mediated mono- and diadduct formations have been determined to take place via singlet-triplet crossings and conical intersections, correlated with the initially promoted triplet or singlet states in different possible reactive orientations. Pyroneside monoadducts are suggested to be formed in the triplet manifold of the system, and to be less prone to yield diadducts because of the properties of the monoadduct low…

On the low-lying singlet excited states of styrene: a theoretical contribution

The present contribution analyses the trans–cis photoisomerization mechanism of ethene and styrene on the singlet manifold. Within the framework of multiconfigurational second-order perturbation theory (CASPT2), the extended multistate approach (MS-CASPT2) is found to be flexible enough to describe energy hypersurfaces adiabatically. For ethene, torsion about the CC bond towards a perpendicular structure leads to a situation where the energy difference between the ground and the lowest excited state is still too large (2.5 eV) for efficient radiationless decay. However, the energy gap decreases to 0.4 eV when one of the methylene moieties is, in addition, pyramidalized from the twisted stru…

ChemInform Abstract: Excitation of Nucleobases from a Computational Perspective I: Reaction Paths

The main intrinsic photochemical events in nucleobases can be described on theoretical grounds within the realm of non-adiabatic computational photochemistry. From a static standpoint, the photochemical reaction path approach (PRPA), through the computation of the respective minimum energy path (MEP), can be regarded as the most suitable strategy in order to explore the electronically excited isolated nucleobases. Unfortunately, the PRPA does not appear widely in the studies reported in the last decade. The main ultrafast decay observed experimentally for the gas-phase excited nucleobases is related to the computed barrierless MEPs from the bright excited state connecting the initial Franck…

Theoretical study of the electronic spectrum of p-benzoquinone

The electronic excited states of p-benzoquinone have been studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) basis sets. The calculation of the singlet–singlet and singlet–triplet transition energies comprises 19 valence singlet excited states, 4 valence triplet states, and the singlet 3s,3p, and 3d members of the Rydberg series converging to the first four ionization limits. The computed vertical excitation energies are found to be in agreement with the available experimental data. Conclusive assignments to both valence and Rydberg states have been performed. The main features of the electronic spectrum correspond to the π…

Essential on the Photophysics and Photochemistry of the Indole Chromophore by Using a Totally Unconstrained Theoretical Approach

Indole is a chromophore present in many different molecules of biological interest, such as the essential amino acid tryptophan and the neurotransmitter serotonin. On the basis of CASPT2//CASSCF quantum chemical calculations, the photophysical properties of the system after UV irradiation have been studied through the exploration of the potential energy hypersurfaces of the singlet and triplet low-lying valence excited states. In contrast to previous studies, the present work has been carried out without imposing any restriction to the geometry of the molecule (C1 symmetry) and by performing minimum energy path calculations, which is the only instrument able to provide the lowest-energy evo…

A Theoretical Study on the Low-Lying Excited States of 2,2′:5′,2′′-Terthiophene and 2,2′:5′,2′′:5′′,2′′′-Quaterthiophene

The nature and properties of the low-lying singlet and triplet valence excited states of 2,2':5',2-terthiophene (terthiophene) and 2,2':5',2:5,2'-quictterthiophene (tetrdthiophene) are discussed on the basis of high-level ab initio. computations. The spectrascopic features determined experimentally for short α-oligothiophenes are rationalised on theoretical grounds. Special attention is devoted to the nonradiative decay process through intersystem crossing (ISC) from the singlet to the triplet manifold, which is known to be relatively less efficient in tetrathiophene. Along the geometry relaxation of the S 1 state of terthiophene, the S' 1 and T 2 states become degenerate, which leads. to a…

Electronic Transitions in Tetrathiafulvalene and Its Radical Cation:  A Theoretical Contribution

The low-lying electronic states of tetrathiafulvalene (TTF) and its radical cation (TTF+) have been studied using the multistate extension of a multiconfigurational second-order perturbation method (MS−CASPT2). The minimum-energy equilibrium geometries optimized at the CASSCF level have a boatlike conformation for the neutral molecule, with no significant barrier toward planarity. A more aromatic planar structure is, however, found for the ionic system. For TTF, the calculations of the vertical excitation energies comprise valence singlet and triplet states as well as the lowest members of the Rydberg series converging to the first ionization limit. Valence doublet states have been consider…

ChemInform Abstract: A Theoretical Determination of the Dissociation Energy of the Nitric Oxide Dimer.

Multi-reference CI methods have been applied to determine the dissociation energy and structure of thecis-N2O2 molecule. The convergence of the theoretical result has been checked with respect to a systematic expansion of the one-electron basis set and the multi-reference CI wave function. The best calculated value, 13.8 kJ/mol, is in agreement with the experimental value, 12.2 kJ/mol. It has been obtained with an extended ANO-type basis set [6s5p3d2f], including the effect of the basis set superposition error (BSSE) in the geometry optimization, and additional effects, such as the electron correlation of core electrons and relativistic corrections, using the average coupled pair functional…

A theoretical study of the electronic spectrum of bithiophene

The electronic spectrum of bithiophene in the energy range up to 6.0 eV has been studied using multiconfigurational second order perturbation theory (CASPT2) and a basis set of ANO type, with split valence quality and including polarization functions on all heavy atoms. Calculations were performed at a planar (trans) and twisted geometry. The calculated ordering of the excited singlet states is 1Bu, 1Bu, 1Ag, 1Ag, and 1Bu with 0–0 transition energies: 3.88, 4.15, 4.40, 4.71, and 5.53 eV, respectively. The first Rydberg transition (3s) has been found at 5.27 eV. The results have been used in aiding the interpretation of the experimental spectra, and in cases where a direct comparison is poss…

Quantum chemical study on the population of the lowest triplet state of psoralen

Abstract The efficient population of the low-lying triplet ππ * state of psoralen is studied with the quantum chemical CASPT2 method. Minima, singlet–triplet crossings, conical intersections, and reaction paths on the low-lying singlet and triplet states hypersurfaces of the system have been computed together with electronic energy gaps and spin–orbit coupling terms. A mechanism is proposed, favorable in the gas phase, for efficient deactivation of the initially populated singlet excited ππ * state, starting with an intersystem crossing with an n π * triplet state and evolving via a conical intersection toward the final lowest-lying ππ * triplet state, protagonist of the reactivity of psora…

Singlet-Triplet States Interaction Regions in DNA/RNA Nucleobase Hypersurfaces.

The present study provides new insight into the intrinsic mechanisms for the population of the triplet manifold in DNA nucleobases by determining, at the multiconfigurational CASSCF/CASPT2 level, the singlet-triplet states crossing regions and the main decay paths for their lowest singlet and triplet states after near-UV irradiation. The studied singlet-triplet interacting regions are accessible along the minimum energy path of the initially populated singlet bright (1)ππ* state. In particular, all five natural DNA/RNA nucleobases have, at the end of the main minimum energy path and near a conical intersection of the ground and (1)ππ* states, a low-energy, easily accessible, singlet-triplet…

Theoretical study of the electronic spectrum of magnesium-porphyrin

Multiconfigurational self-consistent field (SCF) and second order perturbation methods have been used to study the electronic spectrum of magnesium-porphyrin (MgP). An extended ANO-type basis set including polarization functions on all heavy atoms has been used. Four allowed singlet states of E1u symmetry have been computed and in addition a number of forbidden transitions and a few triplet states. The results lead to a consistent interpretation of the electronic spectrum, where the Q band contains one transition, the B band two, and the N band one. The computed transition energies are consistently between 0.1 and 0.5 too low compared to the measured band maxima. The source of the discrepan…

Proton/Hydrogen Transfer Mechanisms in the Guanine–Cytosine Base Pair: Photostability and Tautomerism

Proton/hydrogen-transfer processes have been broadly studied in the past 50 years to explain the photostability and the spontaneous tautomerism in the DNA base pairs. In the present study, the CASSCF/CASPT2 methodology is used to map the two-dimensional potential energy surfaces along the stretched NH reaction coordinates of the guanine–cytosine (GC) base pair. Concerted and stepwise pathways are explored initially in vacuo, and three mechanisms are studied: the stepwise double proton transfer, the stepwise double hydrogen transfer, and the concerted double proton transfer. The results are consistent with previous findings related to the photostability of the GC base pair, and a new contrib…

A theoretical determination of the electronic spectrum of formaldehyde

The electronically excited states of formaldehyde are examined by means of multiconfigurational second-order perturbation (CASPT2) theory with extended ANO-type basis sets. The calculations comprised five valence excited states plus all singlet 3s, 3p, and 3d members of the Rydberg series converging on the first ionization. The computed vertical excitation energies were found to be within 0.2 eV of the available experimental energies. Full geometry optimization has been performed for five valence excited states. Assuming a planar geometry, the “0-0” transition for the valence1A1(π → π*) state is calculated to appear near 7.9 eV, close to the (n y → 3p) region. This state is, however, not pl…

Theoretical Study of the Electronic Excited States of Tetracyanoethylene and Its Radical Anion

The low-lying electronic states of tetracyanoethylene (TCNE) and its radical anion were studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) basis sets. The results obtained yield a full interpretation of the electronic absorption spectra, explain the spectral changes undergone upon reduction, give support to the occurrence of a bound excited state for the anionic species, and provide valuable information for the rationalization of the experimental data obtained with electron transmission spectroscopy.

Multiconfigurational Perturbation Theory: Applications in Electronic Spectroscopy

Electronic Spectra of 2,2‘-Bithiophene and 2,2‘:5‘,2‘ ‘-Terthiophene Radical Cations:  A Theoretical Analysis

Analysis of the electronic spectra of 2,2‘-bithiophene and 2,2‘:5‘,2‘ ‘-terthiophene radical cations has been performed by using multiconfigurational second-order perturbation theory (CASPT2). Atomic natural orbital (ANO) type basis sets of split valence quality, including polarization functions on all heavy atoms, have been used. In agreement with experimental data, in the energy range below the lowest optically allowed transition of the respective neutral system, theoretical results predict two main absorption bands for both cations. The 22Au and 32Au states computed at 1.94 and 2.80 eV, respectively, are related to the corresponding band maxima recorded for bithiophene cation. The 12B1 s…

A theoretical study of the electronic spectrum of styrene

Abstract The electronic spectrum of styrene has been studied by using multiconfigurational second-order perturbation theory (CASPT2). The study includes geometry optimization of the ground state, the lowest triplet state, and the lowest singlet excited state. The covalent 2 1 A ′ state placed vertically at 4.34 eV is assigned to the first band. The 1 1 A ′→3 1 A ′ transition located at 4.97 eV is responsible for the second band. The most intense feature involves the 5 1 A ′ state, which is calculated to lie 6.19 eV above the ground state. The lowest singlet–singlet Rydberg transition (3s) is predicted to occur at 5.85 eV.

The internal rotational barrier of biphenyl studied with multiconfigurational second-order perturbation theory (CASPT2)

A detailedab initio study of the molecular structure and rotational barriers of biphenyl has been performed. First, non-dynamical correlation effects involving the π system are taken into account at the CASSCF level. These wave functions are subsequently employed as reference functions in a multiconfigurational second-order perturbation treatment (CASPT2). The performance single-reference approaches is in addition analysed. The molecular geometries of biphenyl in twisted, coplanar, and perpendicular conformations have been optimized at the CASSCF level. A rotational angle of 44.3° is predicted for the minimum energy conformer in agreement with gas-phase electron diffraction data (44.4±1.2°)…

A Theoretical Study of the Low-Lying Excited States of trans- and cis-Urocanic Acid

A multiconfigurational second-order perturbation theory (CASPT2) study of the lowest lying states in the gas-phase electronic spectra of trans- and cis-urocanic acid is presented. Geometries of both isomers have been optimized at the MP2/6-31G(d) and π-CASSCF/ANO-L(4s3p1d,2s) levels of theory. The geometries are found to differ considerably between the two levels. The vertical and 0−0 excitation spectra were calculated for each isomer. Both singlet and triplet states are described for each, including the lowest lying ππ* excitations and the nOπ* excitations. Remarkably, in the trans spectrum, it is found that the nOπ* state has a higher vertical excitation energy than the lowest ππ* (5.12 v…

A theoretical study of the electronic spectrum of thiophene

Abstract The electronic spectrum of thiophene has been studied using multiconfiguration second-order perturbation theory and extended ANO basis sets. The calculations comprise four singlet valence excited states and the 3s3p3rd Rydberg series. The lowest triplet states were included and some n-π* and n-σ* states. The results have been used to assign the experimental spectrum below 8.0 eV, with a maximum deviation of about 0.1 eV for vertical transition energies. The calculations place the 2 1A1 valence state at 5.33 eV, below the 1 1B2 valence state at 5.72 eV, and the most intense valence transitions at 6.69 eV (3 1A1) and 7.32 eV (4 1B2) with oscillator strengths 0.19 and 0.39, respective…

An Ab Initio Study on the Mechanism of the Atmospheric Reaction NH2+O3→H2NO+O2

The atmospheric reaction NH 2 +0 3 →H 2 NO+O 2 has been investigated theoretically by using MP2, QCISD, QCISD(T), CCSD(T), CASSCF, and CASPT2 methods with various basis sets. At the MP2 level or theory, the hypersurface of the potential energy (HPES) shows a two step reaction mechanism. Therefore, the mechanism proceeds along two transition states (TS1 and TS2), seperated by an intermediate disignated as Int. However, when the single-reference higler correlated QCISD and the multiconfigurational CASSCF methodologies have been employed, the minimum structure Int and TS2 are not found on the HPES, which thus confirms a direct reaction mechanism. Single-reference high correlated and multiconfi…

Assessment of the Potential Energy Hypersurfaces in Thymine within Multiconfigurational Theory: CASSCF vs. CASPT2

The present study provides new insights into the topography of the potential energy hypersurfaces (PEHs) of the thymine nucleobase in order to rationalize its main ultrafast photochemical decay paths by employing two methodologies based on the complete active space self-consistent field (CASSCF) and the complete active space second-order perturbation theory (CASPT2) methods: (i) CASSCF optimized structures and energies corrected with the CASPT2 method at the CASSCF geometries and (ii) CASPT2 optimized geometries and energies. A direct comparison between these strategies is drawn, yielding qualitatively similar results within a static framework. A number of analyses are performed to assess t…

Theoretical Determination of the Singlet → Singlet and Singlet → Triplet Electronic Spectra, Lowest Ionization Potentials, and Electron Affinity of Cyclooctatetraene

The singlet → singlet and singlet → triplet electronic spectra of cycloocta-1,3,5,7-tetraene are studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbitals (ANOs) basis sets. The observed dipole-allowed features at 4.43, 6.02, and 6.42 eV and the spin-forbidden singlet → triplet bands with maxima at 3.05, 4.05, and 4.84 eV (Frueholz, R. P.; Kuppermann, A. J. Chem. Phys. 1978, 69, 3614) are assigned as the transitions 1 1 A 1 → 1 1 A 2 , 1 1 A 1 → 2 1 B 2 (3p z ), 1 1 A 1 → 3 1 E, and 1 1 A 1 - 1 3 A 2 , 1 1 A 1 → 1 3 E, 1 1 A 1 → 1 3 B 1 , respectively. The lowest (3s) Rydberg singlet and triplet states are placed at 5.58 (2 1 A 1 ) and…

Towards the understanding at the molecular level of the structured-water absorption and fluorescence spectra: a fingerprint of π-stacked water

An intriguing absorption peak around ∼270 nm (4.59 eV) has been recurrently recorded in aqueous solutions of salts, sugars, amino acids, in the free-solute zone (exclusion zone) adjacent to various hydrophilic surfaces, as well as a transient in the conversion process of ice to water. The corresponding associated fluorescence has been observed in the interval 480–490 nm (2.58–2.53 eV). The spectroscopic features have been related to the presence of structured water but its nature remains incompletely understood. On the basis of high-level ab initio computations, the main absorption feature of structured water is assigned to the presence of two π-stacked ground-state water molecules, prefera…

A combined theoretical and experimental determination of the electronic spectrum of acetone

A combined ab initio and experimental investigation has been performed of the main features of the electronic spectrum of acetone. Vertical transition energies have been calculated from the ground to the ny→π∗, π→π∗, σ→π∗, and the n=3 Rydberg states. In addition, the 1A1 energy surfaces have been studied as functions of the CO bond length. The 1A1 3p and 3d states were found to be heavily perturbed by the π→π∗ state. Resonant multiphoton ionization and polarization‐selected photoacoustic spectra of acetone have been measured and observed transitions were assigned on internal criteria. The calculated vertical transition energies to the ny→π∗ and all Rydberg states were found to be in agreeme…

The Low-Lying Excited States of 2,2′-Bithiophene: A Theoretical Analysis

The low-energy region of the singlet →singlet, singlet →triplet, and triplet→triplet electronic spectra of 2,2'-bithiophene are studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbitals (ANO) basis sets. The computed vertical, adiabatic, and emission transition energies are in agreement with the available experimental data. The two lowest singlet excited states, 1 1 B u and 2'B u , are computed to be degenerate, a novel feature of the system to be borne in mind during the rationalization of its photophysics. As regards the observed high triplet quantum yield of the molecule, it is concluded that the triplet states 2 3 A g and 2 3 B u ,…

The family of furocoumarins: Looking for the best photosensitizer for phototherapy

Abstract Furocoumarins are widely used as photosensitizers in photochemical therapies against different skin disorders such as psoriasis and vitiligo. Absorption of near-UV light by the chromophore triggers a set of photoreactions related to the therapeutic properties of the technique: linkage of a furocoumarin to thymine DNA nucleobases preventing proliferation of pathogenic cells, or generation of highly reactive singlet oxygen in damaged tissues. The family of furocoumarins has been studied in depth for many years seeking a drug having the most remarkable set of properties to act as a photosensitizer. For this purpose, understanding the underlying photochemical mechanisms behind the effe…

On the N1-H and N3-H Bond Dissociation in Uracil by Low Energy Electrons: A CASSCF/CASPT2 Study.

The dissociative electron-attachment (DEA) phenomena at the N1-H and N3-H bonds observed experimentally at low energies (<3 eV) in uracil are studied with the CASSCF/CASPT2 methodology. Two valence-bound π(-) and two dissociative σ(-) states of the uracil anionic species, together with the ground state of the neutral molecule, are proven to contribute to the shapes appearing in the experimental DEA cross sections. Conical intersections (CI) between the π(-) and σ(-) are established as the structures which activate the DEA processes. The N1-H and N3-H DEA mechanisms in uracil are described, and experimental observations are interpreted on the basis of two factors: (1) the relative energy of …

Adenine and 2-aminopurine: Paradigms of modern theoretical photochemistry

Distinct photophysical behavior of nucleobase adenine and its constitutional isomer, 2-aminopurine, has been studied by using quantum chemical methods, in particular an accurate ab initio multiconfigurational second-order perturbation theory. After light irradiation, the efficient, ultrafast energy dissipation observed for nonfluorescent 9H-adenine is explained here by the nonradiative internal conversion process taking place along a barrierless reaction path from the initially populated 1 (ππ* L a ) excited state toward a low-lying conical intersection (CI) connected with the ground state. In contrast, the strong fluorescence recorded for 2-aminopurine at 4.0 eV with large decay lifetime …

Theoretical Studies of the Electronic Spectra of Organic Molecules

The complete active space (CAS) SCF method in conjunction with multiconfigurational second-order perturbation theory (CASPT2) has been used to study the electronic spectra of a large number of molecules. The wave functions and the transition properties are computed at the CASSCF level, while dynamic correlation contributions to the excitation energies are obtained through the perturbation treatment. The methods yield energies, which are accurate to at least 0.2 eV, except in a few cases, where the CASSCF reference function does not characterize the electronic state with sufficient accuracy. The applications comprise: the polyenes from ethene to octatetraene (cis- and trans-forms); a number …

A theoretical study of the electronic spectrum of cis-stilbene

Abstract The valence and Rydberg electronic excited singlet states of cis -stilbene have been studied using multiconfigurational second-order perturbation theory (CASPT2). The study includes a geometry determination of the ground state. The geometry of the lowest 1 B state has also been optimized, leading to a structure where the two benzyl groups are nearly perpendicular. The calculated vertical spectrum is in agreement with the occurrence of three main bands in the observed one-photon absorption spectrum. Below the relatively intense transition to the 2 1 B (HOMO→LUMO) state at 4.61 eV, two weak transitions have been computed at 4.11–4.45 eV, which correspond to transitions to the 1 1 B a…

Ab initio determination of the ionization potentials of water clusters (H2O)n (n = 2-6).

High-level quantum-chemical ab initio coupled-cluster and multiconfigurational perturbation methods have been used to compute the vertical and adiabatic ionization potentials of several water clusters: dimer, trimer, tetramer, pentamer, hexamer book, hexamer ring, hexamer cage, and hexamer prism. The present results establish reference values at a level not reported before for these systems, calibrating different computational strategies and helping to discard less reliable theoretical and experimental data. The systematic study with the increasing size of the water cluster allows obtaining some clues on the structure and reductive properties of liquid water.

Theoretical Study of the Electronic Spectrum of Imidazole

The complete active space (CAS) self-consistent field (SCF) method and multireference second-order perturbation theory (CASPT2) have been used to study the electronic spectrum of imidazole and the imidazolium ion. The calculations comprise a large number of, both singlet and triplet, valence and Rydberg excited states. A newly developed continuum model has been used to compute solvatochromic shifts. In the gas phase the first and second π → π* excited singlet valence states of imidazole are computed at 6.72 and 7.15 eV, and they shift to 6.32 and 6.53 eV upon solvation. The gas-phase values are somewhat too large (≈0.3 eV) due to an erroneous valence−Rydberg mixing in the CASSCF wave functi…

A theoretical insight into the photophysics of psoralen

Psoralen photophysics has been studied on quantum chemistry grounds using the multiconfigurational second-order perturbation method CASPT2. Absorption and emission spectra of the system have been rationalized by computing the energies and properties of the low-lying singlet and triplet excited states. The S1 ππ* state has been determined to be responsible of the lowest absorption and fluorescence bands and to initially carry the population in the photophysical processes related to the phototherapeutic properties of psoralen derivatives. The low-lying T1 ππ* state is, on the other hand, protagonist of the phosphorescence, and its prevalent role in the reactivity of psoralen is suggested to b…

Photoinduced Formation Mechanism of the Thymine−Thymine (6−4) Adduct

The photoinduced mechanism leading to the formation of the thymine-thymine (6-4) photolesion has been studied by using the CASPT2//CASSCF approach over a dinucleotide model in vacuo. Following light absorption, localization of the excitation on a single thymine leads to fast singlet-triplet crossing that populates the triplet (3)(nπ*) state of thymine. This state, displaying an elongated C(4)═O bond, triggers (6-4) dimer formation by reaction with the C(5)═C(6) double bond of the adjacent thymine, followed by a second intersystem crossing, which acts as a gate between the excited state of the reactant and the ground state of the photoproduct. The requirement of localized excitation on just …

On the Intrinsic Population of the Lowest Triplet State of Thymine

The population of the lowest triplet state of thymine after near-UV irradiation has been established, on the basis of CASPT2//CASSCF quantum chemical calculations, to take place via three distinct intersystem crossing mechanisms from the initially populated singlet bright 1pipi* state. Two singlet-triplet crossings have been found along the minimum-energy path for ultrafast decay of the singlet state at 4.8 and 4.0 eV, involving the lowest 3npi* and 3pipi* states, respectively. Large spin-orbit coupling elements predict efficient intersystem crossing processes in both cases. Another mechanism involving energy transfer from the lowest 1npi* state with much larger spin-orbit coupling terms ca…

On the hexagonal ice-like model of structured water: Theoretical analysis of the low-lying excited states

Abstract The basic molecular unit ( BMU ) of the hexameric ice-like model has been identified as the negatively charged radical species [H 19 O 10 ] − . On the basis of high-level ab initio computations, by means of using the CASPT2 methodology, the low-lying excited states of BMU and its π-stacked dimer are analysed. It is concluded that the BMU constitutes certainly a versatile unit having two spectroscopic features in the near infrared region, two in the visible range and two in the near ultraviolet zone. The main absorption feature is found however for the π-stacked dimer formed by two BMUs , the H 38 O 20 system, which has a pronounced stabilization in the ground state with respect to …

Theoretical studies on the spectroscopy of the 7-azaindole monomer and dimer

The absorption and the emission spectra, both fluorescence and phosphorescence, of the 7-azaindole molecule have been studied by means of the complete active space (CAS) SCF method and multiconfigurational second-order perturbation theory (CASPT2). Excitation energies, oscillator strengths, dipole moments, transition dipole moments, and their directions have been computed and the results compared to those of analogous molecules such as indene, indole, and benzimidazole, to get a homogeneous picture of the photophysics of the systems. The absorption and emission of the 7-azaindole dimer and its related tautomer have also been computed in order to get further insight into the double fluoresce…

Excitation of Nucleobases from a Computational Perspective I: Reaction Paths

The main intrinsic photochemical events in nucleobases can be described on theoretical grounds within the realm of non-adiabatic computational photochemistry. From a static standpoint, the photochemical reaction path approach (PRPA), through the computation of the respective minimum energy path (MEP), can be regarded as the most suitable strategy in order to explore the electronically excited isolated nucleobases. Unfortunately, the PRPA does not appear widely in the studies reported in the last decade. The main ultrafast decay observed experimentally for the gas-phase excited nucleobases is related to the computed barrierless MEPs from the bright excited state connecting the initial Franck…

Theoretical characterization of the absorption spectra of phenanthrene and its radical cation

The vertical absorption spectra of phenanthrene and its radical cation have been studied theoretically by means of a multiconfigurational second-order perturbation approach. Singlet-singlet transition energies and oscillator strengths, and singlet-triplet excitation energies have been studied in the absorption spectrum of phenanthrene up to 6 eV. The absorption spectrum of the pehnanthrene radical cation has been computed up to 3.4 eV. The results obtained confirm previous assignments and also lead to new interpretations of the main features of the spectra of these systems.

Theoretical Analysis of the Electronic Spectra of Benzaldehyde

The electronic spectrum of benzaldehyde has been studied by using multiconfigurational second-order perturbation theory through the multistate extension (MS-CASPT2). The nπ* 11A‘ ‘ state, placed ve...

Selected dissociation‐ and correlation‐consistent configuration interaction by a perturbative criterion

We propose a perturbative criterion to select the most important dissociation‐ or correlation‐consistent type of contributions to perform generalized valence bond‐configuration interaction (GVB‐CI) calculations, dissociation‐consistent configuration interaction (DCCI) or correlation‐consistent configuration interaction (CCCI) approach, respectively. The procedure presented is computationally less demanding than the CCCI proposed by Goddard and co‐workers. To ensure the distance consistency of the MOs used, the nonvalence virtual orbitals are obtained by a projection technique. The results obtained for a few test calculations show the ability of the suggested approach to get close results to…

Photostability and Photoreactivity in Biomolecules: Quantum Chemistry of Nucleic Acid Base Monomers and Dimers

The great potentials of high-level ab initio methods, in particular, the CASPT2//CASSCF protocol, are fully illustrated through: (i) the study of ultrafast energy relaxation in DNA/RNA base monomers, (ii) the intrinsic population mechanism of the lowest triplet state, and (iii) how bioexcimers can be considered as precursors of charge transfer and photoinduced reactivity. In order to describe these processes properly, the presence of conical intersections (CIs) and the topology of the involved pathways have to be determined correctly. Thus, in theoretical calculations the dynamic electronic correlation has to be considered. The accessibility of the CIs (or the seam of CIs) becomes crucial t…

Theoretical Study of the Twisted Intramolecular Charge Transfer in 1-Phenylpyrrole

Ab initio results for the electronic spectra of 1-phenylpyrrole are presented. Vertical, emission, and nonvertical excitation energies have been computed using multiconfigurational second-order per...

Theoretical study of the electronic spectra ofcis-1,3,5-hexatriene andcis-1,3-butadiene

The electronic spectra forcis-1,3-butadiene andcis-1,3,5-hexatriene have been studied using multiconfiguration second-order perturbation theory (CASPT2) and extended ANO basis sets. The calculations comprise all singlet valence excited states below 8.0 eV, the first 3s, 3p, 3d Rydberg states, and the second 3s state. The four lowest triplet states were also studied. The resulting excitation energies forcis-hexatriene have been used in an assignment of the experimental spectrum, leading to a maximum deviation of 0.13 eV for the vertical transition energies. The calculations place the 11 B 2 state 0.04 eV below the 21 A 1 state. 16 excited states were studied incis-butadiene, using a CASPT2 o…

An ab initio study of the electron affinity of O2

Abstract Coupled pair functional, multiconfigurational second-order perturbation theory, and multireference CI methods have been applied in a calculation of the electron affinity of the oxygen molecule. The convergence of the theoretical result has been checked with respect to a systematic expansion of the one-electron basis and the multireference CI wavefunction. The best calculated value, 0.39 eV, is 0.06 eV smaller than the recent experimental value 0.45±0.01 eV.

Interpretation of the electronic absorption spectrum of free base porphin by using multiconfigurational second-order perturbation theory

Abstract Multiconfigurational second-order perturbation (CASPT2) calculations have been performed on the low-lying optically allowed valence excited states of the free base porphin molecule in order to assign the four lowest bands of the spectrum. The low-lying triplet states have also been characterized. A basis set of the atomic natural orbital type of split-valence plus polarization quality for first-row atoms has been employed. Polarization functions are important for an accurate description of the transitions. These CASPT2 results provide a consistent picture of the experimental spectrum. Each band of the spectrum up to 4.5 eV is composed of a pair of states, which become degenerate in…

Are the five natural DNA/RNA base monomers a good choice from natural selection?

In order to prevent the damaging effects of sun radiation in the genetic material, its constituent chromophores, the five natural DNA/RNA nucleobases cytosine, thymine, uracil, adenine, and guanine, should be able to efficiently dissipate absorbed radiation, UV specifically, avoiding as much as possible photoreactions leading to lesions. It has been established experimentally and theoretically that efficient internal conversion channels, still open and relevant in the oligomer-stacked strands, exist in the monomers allowing an effective waste of the initial energy. Previous evidences cannot explain, however, why minor differences in the molecular structure modify drastically the photochemis…

A Three-State Model for the Photophysics of Adenine

An ab initio theoretical study at the CASPT2 level is reported on minimum energy reaction paths, state minima, transition states, reaction barriers, and conical intersections on the potential energy hypersurfaces of two tautomers of adenine: 9H- and 7H-adenine. The obtained results led to a complete interpretation of the photophysics of adenine and derivatives, both under jet-cooled conditions and in solution, within a three-state model. The ultrafast subpicosecond fluorescence decay measured in adenine is attributed to the low-lying conical intersection (gs/pipi* La)(CI), reached from the initially populated 1(pipi* La) state along a path which is found to be barrierless only in 9H-adenine…

Ab Initio Methods for Excited States

This chapter focuses mainly on the performance of ab initio methods for the description of spectroscopic molecular properties of compounds. Most of the quantum-chemical methods developed up to date are based on the concept of the one-electron wave function. The electronic states of a system with N electrons are described by a double expansion. Molecular orbitals (MOs) are one-electron wave functions expressed as linear combinations of a known one-electron basis set (K) and the N electron wave function is formulated in a many-electron basis set formed by determinants (or linear combination of them to form spin-adapted wave functions), built as normalized antisymmetric products of MOs. Accord…

Ultrafast electron transfer in photosynthesis: reduced pheophytin and quinone interaction mediated by conical intersections.

The mechanism of electron transfer (ET) from reduced pheophytin (Pheo(-)) to the primary stable photosynthetic acceptor, a quinone (Q) molecule, is addressed by using high-level ab initio computations and realistic molecular models. The results reveal that the ET process involving the (Pheo(-) + Q) and (Pheo + Q(-)) oxidation states can be essentially seen as an ultrafast radiationless transition between the two hypersurfaces taking place via conical intersections (CIs). According to the present findings, an efficient ultrafast ET implies that the Pheo- and Q move toward each other in a given preferential parallel orientation, reaching the most effective arrangement for ET at intermolecular…

Applications of level shift corrected perturbation theory in electronic spectroscopy

Abstract Multiconfigurational second-order perturbation theory (CASPT2) with a level shift technique used to reduce the effect of intruder states has been tested for applications in electronic spectroscopy. The following molecules have been studied: formamide, adenine, stilbene, Ni(CO) 4 , and a model compound for the active site in the blue copper protein plastocyanin, Cu(Im) 2 (SH)(SH 2 ) + . The results show that the level shift technique can be used to remove the effects of the intruder states in all these molecules. In some cases a drift in the energies as a function of the level shift is observed, which however is small enough that the normal error bar for CASPT2 excitation energies (…

Computational evidence in favor of a two-state, two-mode model of the retinal chromophore photoisomerization

In this paper we use ab initio multiconfigurational second-order perturbation theory to establish the intrinsic photoisomerization path model of retinal chromophores. This is accomplished by computing the ground state ( S 0 ) and the first two singlet excited-state ( S 1 , S 2 ) energies along the rigorously determined photoisomerization coordinate of the rhodopsin chromophore model 4- cis -γ-methylnona-2,4,6,8-tetraeniminium cation and the bacteriorhodopsin chromophore model all- trans -hepta-2,4,6-trieniminium cation in isolated conditions. The computed S 2 and S 1 energy profiles do not show any avoided crossing feature along the S 1 reaction path and maintain an energy gap &gt;20 kcal⋅…

A theoretical study of the electronic spectrum of biphenyl

Abstract The electronic spectrum of biphenyl in the energy range up to 6.0 eV has been studied using multiconfigurational second-order perturbation theory (CASPT2) and a basis set of ANO type, including polarization functions on all carbon atoms. The calculated spectrum gives conclusive assignments to all valence excited singlet states and the low-lying triplet states. The change of the torsional angle between the two benzene rings in the different excited states is shown to be of considerable importance and explains the different excitation energies observed in the gas phase as compared to solution or crystalline biphenyl at low temperatures. The intense transition to the 1 1B1u state is m…

On the theoretical determination of the electron affinity of ozone

Multiconfigurational electron correlation methods have been analyzed in order to theoretically compute the electron affinity (EA) of ozone. The near-degeneracy correlation effects, which are so important in O3 and O 3 − , have been described using complete active space (CAS) SCF wave functions. Remaining dynamic correlation effects are computed using second-order perturbation theory (the CASPT2 method). The best calculated adiabatic value (including zero-point energy corrections), 2.19 eV, is about 0.09 eV larger than the experimental value. Comparative studies using size-consistent coupled pair functional approaches (CPF and ACPF) have also been performed. The harmonic frequencies in O 3 −…

On the photophysics and photochemistry of the water dimer

The photochemistry of the water dimer irradiated by UV light is studied by means of the complete active space perturbation theory//complete active space self-consistent field (CASPT2//CASSCF) method and accurate computational approaches like as minimum energy paths. Both electronic structure computations and ab initio molecular dynamics simulations are carried out. The results obtained show small shifts relative to a single water molecule on the vertical excitation energies of the dimer due to the hydrogen bond placed between the water donor (W(D)) and the water acceptor (W(A)). A red-shift and a blue-shift are predicted for the W(D) and W(A), respectively, supporting previous theoretical a…

Electrostatic control of the photoisomerization efficiency and optical properties in visual pigments: on the role of counterion quenching.

Hybrid QM(CASPT2//CASSCF/6-31G*)/MM(Amber) computations have been used to map the photoisomerization path of the retinal chromophore in Rhodopsin and explore the reasons behind the photoactivity efficiency and spectral control in the visual pigments. It is shown that while the electrostatic environment plays a central role in properly tuning the optical properties of the chromophore, it is also critical in biasing the ultrafast photochemical event: it controls the slope of the photoisomerization channel as well as the accessibility of the S(1)/S(0) crossing space triggering the ultrafast decay. The roles of the E113 counterion, the E181 residue, and the other amino acids of the protein pock…

A CI study of the CuCO and CuCO+ complexes

MO CI calculations are carried out using an optimal space of valence virtual MOs obtained by means of a projection technique, as a linear combination of the AOs which are more occupied in the molecular Fock space. Localization of the occupied MOs and nonvalence virtual MOs is also achieved. The overall procedure is proven to be quite advantageous and well suited to obtain potential energy curves which keep the same physical meaning along the range of distances studied. Using a slightly better than double‐zeta quality basis set, a valence CAS‐CI, and selected CI wave function by the CIPSI algorithm have revealed a possible weak van der Waals interaction for the 2Σ+ state of CuCO, which remai…

A theoretical study of the low-lying states of the anionic and protonated ionic forms of urocanic acid

A multistate second-order perturbation theory (MS−CASPT2) study of the lowest lying states in the electronic spectra of urocanic acid in vacuo is presented. The anionic trans and cis isomers, as well as the biologically important trans protonated ionic structure, are considered. The vertical and 0−0 excitation spectra were computed for each system at the MS−CASPT2/ANO-L level, describing the lowest lying ππ* and nπ* singlet and triplet states. In all three systems, a weakly absorbing ππ* singlet state was observed at ∼4.0 eV in the vertical excitation spectrum, suggesting both a novel assignment and an alternative explanation for the previously described wavelength dependent photochemistry …

Photochemical Nitration by Tetranitromethane. Part XL. Regiochemistry of Trinitromethyl Attachment in the Photolysis of Benzofuran with Tetranitromethane.

Complete-active-space second-order perturbation theory (CASPT2//CASSCF) study of the dissociative electron attachment in canonical DNA nucleobases caused by low-energy electrons (0-3 eV).

Low-energy (0-3 eV) ballistic electrons originated during the irradiation of biological material can interact with DNA/RNA nucleobases yielding transient-anion species which undergo decompositions. Since the discovery that these reactions can eventually lead to strand breaking of the DNA chains, great efforts have been dedicated to their study. The main fragmentation at the 0-3 eV energy range is the ejection of a hydrogen atom from the specific nitrogen positions. In the present study, the methodological approach introduced in a previous work on uracil [I. González-Ramírez et al., J. Chem. Theory Comput. 8, 2769-2776 (2012)] is employed to study the DNA canonical nucleobases fragmentations…

Photosensitization and phototherapy with furocoumarins: A quantum-chemical study

Abstract The effect of electromagnetic radiation on biological objects extends from heating to complex photochemistry, and includes DNA alteration, that properly modified in damaged cells may entail beneficial effects. In this regard, psoralen + UV-A (PUVA) therapy, in which furocoumarins, psoralen-like chromophores, are used as photosensitizers and photoreactants with DNA bases, is one of the most promising strategies against a plethora of diseases. Understanding the underlying photochemical mechanisms is crucial to design effective drugs without undesired side effects. We have undertaken a quantum-mechanical study on the photophysics and photochemistry of furocoumarins, analyzing firstly …

Unified model for the ultrafast decay of pyrimidine nucleobases.

Ultrafast decay processes detected after absorption of UV radiation in gas-phase pyrimidine nucleobases uracil, thymine, and cytosine are ascribed to the barrierless character of the pathway along the low-lying 1(pipi*) hypersurface connecting the Franck-Condon region with an out-of-plane distorted ethene-like conical intersection with the ground state. Longer lifetime decays and low quantum yield emission are on the other hand related to the presence of a 1(pipi*) state planar minimum on the S1 surface and the barriers to access other conical intersections. A unified model for the three systems is established on the basis of accurate multiconfigurational CASPT2 calculations, whereas the ef…

Theoretical spectroscopy of organic systems

Abstract The complete active space (CAS) SCF method in conjunction with the multiconfigurational second-order perturbation theory (CASPT2) has been applied to study the electronically excited states of basic organic compounds. As shown in the lecture with a number of examples, the CASPT2 method is capable of yielding accurate results for relative energies and other properties of excited states, provided that flexible one-electron basis sets are employed. The applications comprise an ample range of systems and problems, including polyenes, conjugated and unconjugated dienes, alternant and nonalternant hydrocarbons, polyenals, etc. As a whole these studies enable both qualitative and quantita…

ChemInform Abstract: An ab initio CI Study on the Rotational Barrier of the Allyl Anion.

All-electron and pseudopotential non-empirical calculations have been performed on C 2v and C s (syn, anti) allyl anion conformations. Using a double-zeta valence-shell basis set within the Epstein-Nesbet definition of the unperturbed Hamiltonian, a value about 19 kcal/mol is found for the barrier to rotation of the allyl anion. This value is the theoretical value obtained with greater accuracy, and the lowest one for the rotational barrier.

Electronic Excitation in a Saturated Chain:  An MS-CASPT2 Treatment of the Anti Conformer of n-Tetrasilane

The singlet−singlet electronic spectrum of the anti conformer of n-tetrasilane has been studied using multiconfigurational wave functions (CASSCF), second-order perturbation theory (CASPT2), and its multi-state extension (MS-CASPT2), in conjunction with large ANO-type basis sets including Rydberg functions. The calculations include the 4s, 4p, and 3d members of the Rydberg series converging on the first ionization. Mixing of valence and Rydberg states observed in the CASSCF wave functions is not fully rectified by single-reference CASPT2 theory, whereas the MS-CASPT2 method separates the valence and Rydberg states effectively. At the MS-CASPT2 level, six valence excited states have been fou…

Modeling hole transfer in DNA: Low-lying excited states of oxidized cytosine homodimer and cytosine–adenine heterodimer

Abstract Charge transport in DNA strands has been studied by characterizing on theoretical grounds the lowest doublet states of the oxidized cytosine homodimer (CC + ) and adenine–cytosine heterodimer (CA + ) along the intermolecular separation of the monomers. The high-level quantum-chemical ab initio CASPT2 method and accurate one-electron basis sets have been employed. Both cationic species are found to be bound with comparable binding energies as those of neutral CC and reduced CC − . The results suggest that charge transport in DNA and the distinct photophysical attributes related to the polymer can be described within the framework of a unified theory. A cooperative micro-hopping mech…

Excited states of the water molecule: Analysis of the valence and Rydberg character

The excited states of the water molecule have been analyzed by using the extended quantum-chemical multistate CASPT2 method, namely, MS-CASPT2, in conjunction with large one-electron basis sets of atomic natural orbital type. The study includes 13 singlet and triplet excited states, both valence and 3s-, 3p-, and 3d-members of the Rydberg series converging to the lowest ionization potential and the 3s- and 3p-Rydberg members converging to the second low-lying state of the cation, 1 math. The research has been focused on the analysis of the valence or Rydberg character of the low-lying states. The computation of the 1 math state of water at different geometries indicates that it has a predom…

In remembrance of Dr. Luis Serrano-Andrés

Ab initio determination of the electron affinities of DNA and RNA nucleobases

High-level quantum-chemical ab initio coupled-cluster and multiconfigurational perturbation methods have been used to compute the vertical and adiabatic electron affinities of the five canonical DNA and RNA nucleobases: uracil, thymine, cytosine, adenine, and guanine. The present results aim for the accurate determination of the intrinsic electron acceptor properties of the isolated nucleic acid bases as described by their electron affinities, establishing an overall set of theoretical reference values at a level not reported before and helping to rule out less reliable theoretical and experimental data and to calibrate theoretical strategies. Daniel.Roca@uv.es Manuela.Merchan@uv.es Luis.Se…

Ab initio study on the low-lying excited states of retinal

Ab initio results for the electronic spectrum of all-trans-retinal and its truncated model 3-methyl-all-trans (10-s-cis)-2,4,6,8,10-undecapentaen-1-al are presented. The study includes geometry determination of the ground state. Vertical excitation energies have been computed using multiconfigurational second-order perturbation theory through the CASPT2 formalism. The lowest singlet excited state in gas phase is predicted to be of nπ∗ character. The lowest triplet state corresponds, however, to a ππ∗ state. The most intense feature of the spectrum is due to the strongly dipole-allowed ππ∗ transition, in accordance with the observed maximum in the one-photon spectra. The vertical excitation …

Quantum chemistry of the excited state: 2005 overview

The present contribution contains an overview of quantum-chemical methods and strategies to compute and interpret spectroscopic and photochemical phenomena in molecular systems. The state of the art for the quantum chemistry of the excited state is reviewed, focusing in the advantages and disadvantages of the most commonly employed computational methods, from the single configurational procedures like CI-Singles (CIS), propagator approaches, and Coupled-Cluster (CC) techniques, to the more sophisticated multiconfigurational treatments, with particular emphasis on perturbation theory, the CASPT2 approach. Also, a short summary on the performance, lights, and shadows of the popular TDDFT meth…

On the electronic coupling of the C=O stretching vibrations in Cl(2)Pt(malonate)(-).

Ab initio calculations on the complex Cl(2)Pt(malonate)(-) have been performed to estimate the magnitude and the source of the splitting between the symmetric and antisymmetric C=O stretches. It is shown that a large part of the splitting can result from pure electronic coupling mediated via the pi-structure of the ligand. Calculations have also been performed on the free ligand malonate(-) and the neutral malonaldehyde that show decreasing electronic coupling, which is consistent with the decreasing resonance stabilization. A simple effective C=O interaction potential has been produced for Cl(2)Pt(malonate)(-) that is used to estimate the electronic splitting of the 1600 cm(-1) IR band to …

Determination of the electron-detachment energies of 2'-deoxyguanosine 5'-monophosphate anion: influence of the conformation.

The vertical electron-detachment energies (VDEs) of the singly charged 2'-deoxyguanosine 5'-monophosphate anion (dGMP - ) are determined by using the multiconfigurational second-order perturbation CASPT2 method at the MP2 ground-state equilibrium geometry of relevant conformers. The origin of the unique low-energy band in the gas phase photoelectron spectrum of dGMP - , with maximum at around 5.05 eV, is unambiguously assigned to electron detachment from the highest occupied molecular orbital of π-character belonging to guanine fragment of a syn conformation. The presence of a short H-bond linking the 2-amino and phosphate groups, the guanine moiety acting as proton donor, is precisely resp…

Toward the understanding of DNA fluorescence: The singlet excimer of cytosine

By using the multiconfigurational second-order perturbation method CASPT2, including corrections for the basis set superposition error, the lowest-singlet excited state of the face-to-face π-stacked cytosine homodimer is revealed to be bound by about half an eV, being the source of an emissive feature consistent with the observed redshifted fluorescence. Gloria.Olaso@uv.es Daniel.Roca@uv.es Luis.Serrano@uv.es Manuela.Merchan@uv.es

A Theoretical Determination of the Low-lying Electronic States of the p-Benzosemiquinone Radical Anion

The low-lying electronic states of the p-benzosemiquinone radical anion are studied using multiconfigurational second-order perturbation theory (CASPT2) and extended atomic natural orbital (ANO) ba...

CCDC 912917: Experimental Crystal Structure Determination

Related Article: Vicenta Saurí, Josep M. Oliva, Drahomír Hnyk, Jonathan Bould, Jakub Braborec, Manuela Merchán, Pavel Kubát, Ivana Císařová, Kamil Lang, and Michael G. S. Londesborough|2013|Inorg.Chem.|52|9266|doi:10.1021/ic4004559