Search results for "iTED"

Quantum size effects in ambient CO oxidation catalysed by ligand-protected gold clusters

2009

Finely dispersed nanometre-scale gold particles are known to catalyse several oxidation reactions in aerobic, ambient conditions. The catalytic activity has been explained by various complementary mechanisms, including support effects, particle-size-dependent metal-insulator transition, charging effects, frontier orbital interactions and geometric fluxionality. We show, by considering a series of robust and structurally well-characterized ligand-protected gold clusters with diameters between 1.2 and 2.4 nm, that electronic quantum size effects, particularly the magnitude of the so-called HOMO-LUMO energy gap, has a decisive role in binding oxygen to the nano-catalyst in an activated form. T…

A MRCI PS and CASSCF study of the ground state MgO dissociation energy

1991

Abstract Ab initio calculations at CASSCF and MRCI PS levels are used to determine the dissociation energy for the X 1 Σ + state of MgO, which adiabatically dissociates to the ground state 1 S g of magnesium and to the excited 1 D g state of oxygen, as well as other spectroscopic parameters. Emphasis is placed upon the problem of properly selecting an adequate active space in CASSCF calculations and upon the improvements obtained in MRCI by selecting perturbatively the most important contributions to the total wavefunction and evaluating the remaining ones only by perturbational method. Through a procedure based on stabilizing the computed dissociation energy, values of 3.87 eV (MRCI PS) an…

Ground and excited state polarizabilities and dipole transition properties of benzene from coupled cluster response theory

1999

Abstract The electronic properties and transition properties have been investigated for the lowest singlet electronic states of benzene using coupled cluster response theory. The polarizabilities have been calculated for the ground state and the 1 1 B 2u , 1 1 B 1u 1 1 E 1u and 2 1 E 1u excited states. The dipole allowed transitions out of these states have also been calculated and discussed in the context of the calculated polarizabilities. Oscillator strengths and the second electronic moments of the charge distributions have been used to characterize and identify qualitative features of the individual states. The performance of coupled cluster singles (CCS), the recently proposed CC2 mod…

Interplay between spin crossover and proton migration along short strong hydrogen bonds

2021

The iron(ii) salt [Fe(bpp)2](isonicNO)2·HisonicNO·5H2O (1) (bpp = 2,6-bis(pyrazol-3-yl)pyridine; isonicNO = isonicotinate N-oxide anion) undergoes a partial spin crossover (SCO) with symmetry breaking at T1 = 167 K to a mixed-spin phase (50% high-spin (HS), 50% low-spin (LS)) that is metastable below T2 = 116 K. Annealing the compound at lower temperatures results in a 100% LS phase that differs from the initial HS phase in the formation of a hydrogen bond (HB) between two water molecules (O4W and O5W) of crystallisation. Neutron crystallography experiments have also evidenced a proton displacement inside a short strong hydrogen bond (SSHB) between two isonicNO anions. Both phenomena can al…

Magnetic exchange interaction in clusters of orbitally degenerate ions. I. Effective Hamiltonian

2001

Abstract A new effective Hamiltonian is reported for the kinetic exchange between two arbitrary terms 2S A +1 Λ A and 2S B +1 Λ B that can be ground or excited in octahedrally coordinated transition metal ions. This Hamiltonian is applicable to both homo- and heterometallic clusters. For the homonuclear cluster the resonance part of the effective Hamiltonian is also presented for the case when one of the ions is excited. The operator part of the exchange Hamiltonian contains symmetry adapted products of the cubic irreducible tensors acting in orbital spaces ΛA and ΛB and scalar product of site spin operators. The parameters of the Hamiltonian are defined by the relevant intercenter transfer…

The Triplet Excimer of Naphthalene: A Model System for Triplet−Triplet Interactions and Its Spectral Properties

2011

Basic concepts of triplet excimer formation and triplet−triplet interactions between molecules with conjugated π-systems are investigated by means of ab initio quantum chemical calculations, employing the second-order coupled-cluster method CC2 and the second-order propagator method ADC(2). The naphthalene dimer turns out to be a very fruitful model system for which weak and strong electronic coupling can be identified depending on the mutual arrangement of the monomer moieties. From geometry optimizations in the excited state, we determine binding energies, including solvent effects, and transient absorption spectra. The most stable T1 conformation turns out to be a face-to-face arrangemen…

Strong donor–acceptor couplings in a special pair-antenna model

2010

A special pair model composed of two cofacial zinc porphyrins (acceptor) linked to a free base (donor) acts as an energy transfer dyad. Despite the absence of conjugation, ππ*/charge transfer excited states and ultrafast energy transfer (∼5 ps) are noted.

Excitation of Nucleobases from a Computational Perspective I: Reaction Paths

2013

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…

Multiconfigurational Perturbation Theory: Applications in Electronic Spectroscopy

2007

Electron Energy Loss and DFT/SCI Study of the Singlet and Triplet Excited States of Aminobenzonitriles and Benzoquinuclidines:  Role of the Amino Gro…

1999

Spectroscopic consequences of varying the twist angle of the amino group in aminobenzonitrile systems in the electronic ground state are investigated by applying electron energy loss (EEL) spectroscopy and density functional theory to 4-N,N-dimethylaminobenzonitrile (DMABN), 4-N,N-dimethylamino-3,5-dimethylbenzonitrile (MMD), benzoquinuclidine (BQ), and 6-cyanobenzoquinuclidine (CBQ). A number of singlet and triplet excited states was observed and assigned with the help of DFT/SCI theory. The results characterize the gas-phase spectroscopy of the molecules and verify to within 0.3 eV the predictive power of DFT/SCI theory for vertical states over a wide range of twist and pyramidalization a…