0000000000006440
AUTHOR
R. Crespo
Theoretical Design of Organic Metals Based on the Phthalocyanine Macrocycle
Phthalocyanine molecular crystals and cofacially linked polymers are well documented as low-dimensional materials that may attain high electrical conductivities. Air-stable conductivities on the order of 1 to 1000 S/cm after partial oxidation by iodine have been reported.1–3 These conductivity studies indicate that the electrical conductivity has very little dependence on the identity of the atom complexed in the cavity, but is strongly dependent on the orientation and spacing of the phthalocyanine rings. A columnar stacking with minimum spacing leads to a maximum interaction between π-molecular orbitals on adjacent rings and promotes the highest conductivity. More effective π-interactions …
Evidence for an Intermediate in the Methylation of CB11H12−with Methyl Triflate: Comparison of Electrophilic Substitution in Cage Boranes and in Arenes
The trideuteriomethylation of BH vertices in CB11H12− and its derivatives with CD3OTf (OTf=triflate, trifluoromethanesulfonate) yields a mixture of BCD3 and BCHD2 substitution products, thus demonstrating the intermediacy of a species with a long enough lifetime for hydrogen scrambling between the boron vertex and the methyl substituent. No such scrambling is observed if CD3OTf is used to methylate toluene. According to density functional theory calculations, the intermediate in BH vertex methylation is a three-center bonded σ adduct of a methyl cation to the BH bond and the proton scrambling occurs via a transition structure containing a distorted square-pyramidal methane attached axially …
Electronic excitations of 1,4-disilyl-substituted 1,4-disilabicycloalkanes: a MS-CASPT2 study of the influence of cage size.
We present a multistate complete active space second-order perturbation theory computational study aimed to predict the low-lying electronic excitations of four compounds that can be viewed as two disilane units connected through alkane bridges in a bicyclic cage. The analysis has focused on 1,4-disilyl-1,4-disilabicyclo[2.2.1]heptane (1a), 1,4-bis(trimethylsilyl)-1,4-disilabicyclo[2.2.1]heptane (1b), 1,4-disilyl-1,4-disilabicyclo[2.1.1]hexane (2a), and 1,4-bis(trimethylsilyl)-1,4-disilabicyclo[2.1.1]hexane (2b). The aim has been to find out the nature of the lowest excitations with significant oscillator strengths and to investigate how the cage size affects the excitation energies and the…
The Transoid, Ortho, and Gauche Conformers of Decamethyl-n-tetrasilane, n-Si4Me10: Electronic Transitions in the Multistate Complete Active Space Second-Order Perturbation Theory Description
Multistate complete active space second-order perturbation theory (MS-CASPT2) is used to improve earlier descriptions of the low-energy valence excited states of the transoid, ortho, and gauche conformers of decamethyl-n-tetrasilane, n-Si4Me10, using a generally contracted basis set of atomic natural orbitals (ANOs) at a ground-state geometry optimized in the second-order Moller−Plesset perturbation theory (MP2) approximation with Dunning's correlation consistent triple-ζ basis set (cc-pVTZ) on the silicon atoms and the 6-31G* and 6-31G basis sets on the carbon and hydrogen atoms, respectively. Relative energies, relative free energies, and mole fractions of the transoid, ortho, and gauche …
Theoretical Characterization of the Electronic Structure of Poly(Heteroaromatic Vinylenes)
Since the pioneering work of the mid-seventies, the number of organic π-conjugated polymers, that can be made highly conducting upon appropriate chemical treatment, has grown tremendously. Among the most studied of these compounds are those based on fivemembered heteroaromatic rings, such as polythiophene (PT) and polypyrrole (PPy). The attractiveness of these polymers, relative to polyacetylene (PA) and poly(p-phenylene) (PPP), is the high chemical and thermal stability they show.1
Calculation of the conformational dependence of valence and Rydberg states in n-tetrasilane
Abstract Previous CIS/6-31G ∗∗ and CASSCF/6-31G ∗ calculations of valence excited states of n -tetrasilane accounted qualitatively for the observed conformational dependence of condensed-phase UV absorption spectra. In an attempt to understand this result, we have performed CIS calculations with a larger basis set (MC-311G(2d) on Si, 6-311G on H, and 2s, 2p, and 2d diffuse orbitals at molecular center of mass). The first two excited states are of valence character at all dihedral angles if the molecule is isolated. When it is embedded in a rare gas cluster, the lowest four states are of valence character, and the results are nearly identical with those obtained without diffuse orbitals in t…
Theoretical investigation of the electronic structure of fullerenes
Summary form only given. Research in the growing family of fullerene compounds are in continuous expansion. Much theoretical and experimental effort has been devoted to the study of the electronic properties these carbon cages present. However, although significant work has been done, the purpose of this paper is to provide a detailed theoretical analysis of the electronic properties of several fullerenes. We have calculated the electronic structure of these carbon cages within the valence effective Hamiltonian (VEH) approach. The calculated one-electron energy levels are correlated with experimental UPS spectra. The good correlation between calculated and experimental values shows VEH appr…
Influence of Structural Isomerism on the Electronic Properties of Extended Phthalocyanines
Abstract The electronic structure of isomeric naphthalocyanines (H2Nc) is investigated using the nonempirical valence effective Hamiltonian (VEH) technique. The way the outer benzene rings are annelated (linearly as in 2,3-H2Nc or angularly as in 1,2-H2Nc) is shown to determine very important changes in the electronic properties. While similar properties to those of phthalocyanine are calculated for 1,2-H2Nc, lower oxidation potentials and red-shifted optical absorptions are predicted for 2,3-H2Nc in agreement with experimental data. The isomerism resulting from the different relative positions of the angularly annelated benzene rings in 1,2-H2Nc is shown to have almost no effect on the ele…
Evolution of the electronic properties of graded poly(thienylene vinylene)-poly(pyrrylene vinylene) mixed copolymers
Abstract We present a valence effective Hamiltonian (VEH) theoretical investigation of the evolution of the electronic properties of poly(thienylene vinylene)-poly(pyrrylene vinylene) mixed copolymers as a function of the unit cell content on the basis of ab initio 3–21G∗ and 3–21G optimized geometries. As a consequence of the strong localization of the LUCO both electron affinity and bandgap deviate from a linear dependence, and only the ionization potential presents the expected values.
Theoretical prediction of the electronic properties of silicon fullerenes
Summary form only given. High symmetry silicon clusters present currently intense interest because of the possibility they present properties similar to those displayed by fullerenes. Thermodynamic studies have shown that the buckminsterfullerene structure of Si6o is much more stable than other suggested structures. We present here a detailed investigation of the structure and electronic properties of silicon cluster analogous to fullerenes. We have made use of AMI method to obtain reliable geometrical parameters. The calculated valence effective Hamiltonian (VEH) electronic structures are used to predict ionization potentials, electron affinities, HOMO-LUMO energy gaps and first allowed tr…
Theoretical search for low-band-gap vinylene linked polymers: heteroarylene vinylene copolymers
Abstract We report a study of the role of the heteroatoms on the electronic properties of poly(thienylene vinylene-co-furylene vinylene), poly(thienylene vinylene-co-pyrrylene vinylene) and poly(furylene vinylene-co-pyrrylene vinylene) at the VEH level. The theoretical calculations show that the electronic properties depend on the nature of the heteroatoms present in the copolymer. The VEH trends are in accord with experimental and theoretical data reported in the literature.
Measured and Calculated Oxidation Potentials of 1-X-12-Y-CB11Me10– Anions
Cyclic voltammetry of 31 icosahedral carborane anions 1-X-12-Y-CB(11)Me(10)(-) at a Pt electrode in liquid SO(2) revealed a completely reversible one-electron oxidation even at low scan rates, except for the anions with Y = I, which are oxidized irreversibly up to a scan rate of 5.0 V/s, and the anion with X = COOH and Y = H, whose oxidation is irreversible at scan rates below 1.0 V/s. Relative reversible oxidation potentials agree well with RI-B3LYP/TZVPP,COSMO and significantly less well with RI-BP86/TZVPP,COSMO or RI-HF/TZVPP,COSMO calculated adiabatic electron detachment energies. Correlations with HOMO energies of the anions are nearly as good, even though the oxidized forms are subjec…
Theoretical characterization of the electronic properties of unsymmetrical phthalocyanine analogues
Abstract The electronic structure of unsymmetrical phthalocyanine-based compounds is investigated using the nonempirical valence effective Hamiltonian (VEH) method. The VEH results predict a slight but continuous destabilization of the HOMO level and a monotonous narrowing of the HOMO-LUMO energy gap as the size of the system increases. The theoretical trends are found to agree with optical absorption experimental data and show that the optical properties of phthalocyanine can be monitored by adjusting the size of the π-conjugated macrocycle.
Thermochemical analysis of the OH+C2H4→C2H4OH reaction using accurate theoretical methods
Abstract The radical addition of hydroxyl radical to ethene has been investigated using ab initio methods and density functional theory. The structure and energetics of reactants, prereaction complex, transition state structure and radical product have been optimized and vibrational frequencies have been calculated at the HF, MP2, B88, and B3LYP level using 6-31G* and 6-311G** basis sets. The energetics of the reaction is characterized within the MP2, DFT, G2, CBS, and BAC approaches. The evolution of the barrier height and reaction enthalpy has been systematically investigated with respect to the methodological approach employed. The best agreement with experimental results for the barrier…
Theoretical calculations on the valence electronic structure of naphthalocyanine: comparison with experimental photoemission data
Abstract We present a theoretical investigation of the electronic structure of metal-free 2,3-naphthalocyanine (2,3-H2Nc) using the valence effective Hamiltonian (VEH) technique. The VEH-DOVS curves obtained for 2,3-H2Nc are correlated with those calculated for naphthalene molecule and a one-to-one correspondence is found. From the electronic structure standpoint, naphthalocyanine has to be viewed as formed by joining four naphthalene moieties to the central carbon-nitrogen ring. An excellent quantitative agreement is found between VEH-DOVS curves and experimental photoemission data.
Theoretical characterization of the electronic structure of polymeric systems combining disilanylene and /spl pi/-conjugated units
Summary form only given. Polymeric organosilicon systems are in the forefront of research, due to their intriguing properties (semiconducting behavior, photoconduction, non-linear optical properties, and thermo- and piezochromism). The fundamental skeletons of most of these polymers consist of a /spl sigma/-electron network formed by silicon-silicon bonds. Recently, polymeric organosilicon systems containing a regular alternation of a disilanylene unit and carbon /spl pi/-conjugated moiety as phenylene, ethynylene, or ethenylene have been synthesized. The purpose of this contribution is to perform a detailed study of the electronic structure of polymers containing alternated disilanylene an…
Theoretical study of the effect of substituent and backbone conformation on the electronic properties of symmetrically substituted poly(di‐n‐alkylsilanes)
We present the results of ab initio 3‐21G∗ geometry optimizations and valence effective Hamiltonian (VEH) band structure calculations aimed at determining the evolution of the geometric and electronic (ionization potential, electron affinities, and band gaps) properties of all‐trans poly(dimethylsilane), poly(diethylsilane), poly(di‐n‐propylsilane), and poly(di‐n‐butylsilane) when increasing the size of the alkyl group. In the latter polymer, we have also studied the 7/3 conformation, in order to analyze the effect of the backbone conformation on the geometric and electronic structure. The VEH ionization potentials of all‐trans poly(di‐n‐alkylsilanes) are almost equal, and as experimental p…
Electronic properties of poly(pyrrylene vinylenes): a theoretical approach
Abstract We present a theoretical investigation of the electronic properties of poly(pyrrylene vinylene) and its methyl and methoxy derivatives by using the valence effective Hamiltonian (VEH) method. Theoretical calculations predict that the electronic properties of poly(pyrrylene vinylene) remain almost unaffected upon methyl substitution, while a noticeable lowering of ∼0.4 eV is obtained for the ionization potential and energy gap upon methoxy substitution. The VEH trends are in quantitative agreement with experimental data reported for poly(thienylene vinylene) derivatives.
Ab initio study of the internal rotation potential of p-disilanylbenzene and p-(1,1′,2,2′-tetramethyldisilanyl)benzene
Abstract We present a detailed ab initio study of the geometrical structure and torsional potential of p -disilanylbenzene and its methyl derivative p -(1,1′,2,2′-tetramethyldisilanyl)benzene. The geometries of the different conformers have been fully optimized at the Hartree-Fock 3-21G ∗ and 6-31G ∗ levels. In all cases, the most stable conformation corresponds to the conformer in which the phenylene group lies in a plane perpendicular to the four silicon atoms. The torsional potential curves calculated using the 3-21G ∗ and 6-31G ∗ basis sets show similar features. In the case of p -(1,1′,2,2′-tetramethyldisilanyl)benzene the interactions between the hydrogen atoms of phenylene and the me…
Multiconfigurational second-order perturbation calculation of the electronic absorption spectrum of trisilane, Si3H8
The low-lying singlet excited states of trisilane have been computed using the complete active space self-consistent field (CASSCF) method, second-order perturbation theory (CASPT2), and the extended multistate CASPT2 (MS-CASPT2) approach, and a generally contracted basis set of atomic natural orbitals (ANOs) including Rydberg functions. The ground state structure was obtained from an ab initio optimization at the second-order M⊘ller-Plesset perturbation theory (MP2) level using Dunning's correlation-consistent triple-zeta basis set (cc-pVTZ) and agrees well with experiment. The calculation of the electronic transitions included the lower valence excited states and two Rydberg series conver…
Electronic structure of polysilanes: influence of substitution and conformation
Abstract The valence effective Hamiltonian (VEH) quantum-chemical approach is used to investigate the electronic properties of polysilane. The valence band structure calculated for this fully saturated polymer is analyzed in terms of orbital contributions and compared to that of the closely related carbon polymer, polyethylene. The effects of alkyl substitution and silicon backbone conformation are studied by elucidating the modifications that these structural changes induce on the electronic valence band structure of all-trans unsubstituted polysilane. The VEH results predict a decrease of the band gap upon alkyl substitution and on going from helical to all-trans conformations.
Theoretical study on the effect of backbone conformation on the electronic structure of poly(di-n-butylsilane)
Abstract We present the results of valence effective Hamiltonian (VEH) calculations on the electronic structure of poly(di-n-butylsilane) in its all-trans and 7/3 conformations. The band structure of all-trans conformation is analyzed in detail and the effects of the backbone conformation on the electronic and optical properties are studied. The VEH results for the 2/1 and 7/3 conformations are in excellent quantitative agreement with photoemission and UV-absorption data and show the reliability of the VEH method to deal with organopolysilanes.
Electronic structure of poly(p-(disilanylene)phenylene)
Abstract We present the geometrical and electronic structures of several isomers of poly(p-(disilanylene)phenylene), The structural analysis, performed at the 3-21G* level, shows that the isomers with the phenylene group perpendicular to the silicon backbone are the more stable conformations, displaying almost the same energy. The electronic properties, as obtained from the valence-effective Hamiltonian (VEH) band structure calculations, strongly depend on the disposition of the phenylene group into the polymeric backbone. The VEH predicts a wide and asymmetric absorption band in excellent agreement with UV experimental data.
Scope and Limitations of Baird's Theory on Triplet State Aromaticity: Application to the Tuning of Singlet–Triplet Energy Gaps in Fulvenes
Utilizing Baird's theory on triplet state aromaticity, we show that the singlet-triplet energy gaps (DeltaE(ST)) of pentafulvenes are easily varied through substitution by as much as 36 kcal mol(-1). This exploits the fact that fulvenes act as aromatic chameleons in which the dipoles reverse on going from the singlet ground state (S(0)) to the lowest pipi* triplet state (T1); thus, their electron distributions are adapted so as to achieve some aromaticity in both states. The results are based on quantum chemical calculations with the OLYP density functional theory method and the CASPT2 ab initio method, as well as spectroscopic determination of DeltaE(ST) by triplet sensitization. The findi…
VEH electronic band structure of poly(phenylsilane)
Abstract The electronic structure of all-trans syndiotactic and isotactic poly(phenylsilane) has been calculated using the valence effective Hamiltonian (VEH) method. The effects of attachment of the phenyl group on the electronic properties of polysilane are analysed in detail. The VEH results show a decrease of ionization potential and an increase of electron affinity which determine an important reduction of the bandgap. These features are correlated with σ−π and σ ∗ −π ∗ interactions between the silicon backbone and the phenyl group.
MS-CASPT2 analysis of the UV thermochromism of octamethyltrisilane
We interpret the reversal of the direction of the thermochromic shift of the first absorption band of peralkylated oligosilanes as the silicon chain is extended, based on multistate complete active space second-order perturbation theory (MS-CASPT2) calculations for octamethyltrisilane, Si3Me8. The observed shift is attributed to the effect of b1 distortions from ground state equilibrium geometry on vertical excited state energies and intensities. A generally contracted basis set of atomic natural orbitals (ANOs) at a ground state geometry optimized in the second-order Moller–Plesset perturbation theory (MP2) approximation with Dunning's correlation consistent triple-zeta basis set (cc-pVTZ)…
VEH electronic structure of Si60
Abstract We present a valence effective Hamiltonian (VEH) theoretical investigation on the electronic structure of Si60. The VEH one-electron energy level distribution calculated for Si60 is used to predict the ionization potential (7.86 eV), electron affinity (6.62 eV) and HOMO-LUMO energy gap (1.24 eV) of Si60. We also report the first allowed electronic transitions of Si60. The results are compared with other theoretical levels and with those calculated for C60.
Theoretical study of asymmetrically substituted poly(organosilanes)
Summary form only given. Poly(organosilanes) [-SiRR'-] where R and R' represent various alkyl and aryl groups, are the subject of intense scientific and technological interest because of the interesting properties such as, intense near-UV absorption, highly efficient photoluminiscence, non-linear optical properties, these polymers present. In this contribution, we present the electronic structure of some asymmetrically substituted poly(organositanes). We have focused the study on the effect of the tacticity on the electronic structure of the polymer. We have employed ab initio methods to obtain reliable geometrical parameters. The band structure calculations have been performed using the va…
Photoelectron spectrum of poly(methyl-n-propylsilane): a valence band theoretical interpretation
Abstract The results of a theoretical investigation of the photoelectron spectrum of poly(methyl-n-propylsilane) are reported. The simulation of the UPS spectrum has been obtained from band structure calculations performed using the valence effective Hamiltonian method (VEH). The theoretical spectrum is fully consistent with the experimental UPS spectrum, and an excellent quantitative agreement is observed when the energies of the main peaks of both spectra are compared. A complete analysis of the contributions to the main peaks, together with a detailed interpretation of all the photoemission bands observed in the experimental spectrum have also been performed.
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…
Rotational spectra of gauche perfluoro-n-butane, C4F10; perfluoro-iso-butane, (CF3)3CF; and tris(trifluoromethyl)methane, (CF3)3CH
Abstract The microwave spectra of the gauche conformer of perfluoro- n -butane, n -C 4 F 10 , of perfluoro- iso -butane, (CF 3 ) 3 CF, and of tris(trifluoromethyl)methane, (CF 3 ) 3 CH, have been observed and assigned. The rotational and centrifugal distortion constants for gauche n -C 4 F 10 are: A = 1058.11750(7) MHz, B = 617.6832(1) MHz, C = 552.18794(1) MHz, Δ J = 0.0257(5) kHz, δ J = 0.0052(3) kHz. A C–C–C–C dihedral angle, ω , of ∼55° has been determined. These values agree well with those obtained from a coupled cluster (CCSD/cc-PVTZ) calculation. The rotational and centrifugal distortion constants for iso -C 4 F 10 and iso -C 4 HF 9 are: B o = 816.4519(4) MHz, D J = 0.023(2) …
AM1 prediction of the equilibrium geometry of Si60
Abstract AM1 calculations have been carried out to determine the equilibrium geometry of Si 60 . The predicted I h geometry shows that bonds may be clearly identified as single (2.297 A) or aromatic (2.092 A). Several analogies and differences between Si 60 and C 60 are pointed out. Especially remarkable is the bigger size of the Si 60 cluster, which is predicted to have a radius 2 A larger than that of the C 60 cluster. Results are compared to other levels of theory.