A theoretical investigation on the origin dependence of the vibronic polarizabilities, isotropic and anisotropic rotational invariants, and scattering cross sections in Resonance Raman Optical Activity (RROA) spectroscopy is presented. Expressions showing the origin dependence of these polarizabilities were written in the resonance regime using the Franck-Condon (FC) and Herzberg-Teller (HT) approximations for the electronic transition moments. Differently from the far-from-resonance scattering regime, where the origin dependent terms cancel out when the rotational invariants are calculated, RROA spectrum can exhibit some origin dependence even for eigenfunctions of the electronic Hamiltonian. At the FC level, the RROA spectrum is completely origin invariant if the polarizabilities are calculated using a single excited state or for a set of degenerate states. Otherwise, some origin effects can be observed in the spectrum. At the HT level, RROA spectrum is origin dependent even when the polarizabilities are evaluated from a single excited state but the origin effect is expected to be small in this case. Numerical calculations performed for (S)-methyloxirane, (2R,3R)-dimethyloxirane, and (R)-4-F-2-azetidinone at both FC and HT levels using the velocity representation of the electric dipole and quadrupole transition moments confirm the predictions of the theory and show the extent of origin effects and the effectiveness of suggested ways to remove them.

A methodology is presented for the study of fundamental, combination, and overtone Raman transitions, including a treatment based on the contact transformation theory for the mechanical anharmonicity from the cubic potential energy terms. The results obtained for acetylene and its deutered isotopologues show that anharmonicity effects on the Raman intensities can be very strong, particularly in the second‐order transitions. On average, these effects vary from 6.4 to 11.6% for fundamental transitions, from 26.8 to 36.0% for combinations, and from 56.3 to 86.6% for the overtones. Quantitative agreement of the calculated fundamental Raman cross sections with experiment was achieved. For combinations, it was determined that only semiquantitative agreement was achieved, while the computed overtone cross sections overestimated the experimental values. © 2012 Wiley Periodicals, Inc.

The monosignate character of resonance Raman optical activity (RROA) spectra has been often taken as granted in experimental and computational approaches, on the basis of basic theoretical approximations only considering resonance with a single electronic state of the molecule and the scattering process to be governed by the Franck-Condon mechanism. We show in this letter for the first time that, by resorting to a fully quantum mechanical (QM) methodology able to take into account all terms entering the general definition of RROA, and which considers excited state interference and Herzberg-Teller effects, sign alternation and at the same time intensity enhancement in RROA spectra is obtained. Such features constitute an important milestone toward the exploration of RROA of a wide range of chiral biological molecules.

AB INITIO CALCULATION OF DYNAMIC RAMAN INTENSITIES USING THE LINEAR RESPONSE THEORY.In this paper a methodology for the computation of Raman scattering cross-sections and depolarization ratios within the Placzek Polarizability Theory is described. The polarizability gradients are derived from the values of the dynamic polarizabilities computed at the excitation frequencies using ab initio Linear Response Theory. A sample application of the computational program, at the HF, MP2 and CCSD levels of theory, is presented for H 2 O and NH 3 . The results show that high correlated levels of theory are needed to achieve good agreement with experimental data.Keywords: Raman spectra; vibrational spectroscopy; ab initio Linear Response Theory. INTRODUÇÃOApesar da Teoria da Polarizabilidade de Placzek datar de 1934 1 , o estudo teórico de intensidades e perfis de excitação Raman tornouse viável computacionalmente apenas na última década. Atualmente, implementações computacionais para o cálculo de intensidades Raman estáticas são bastante freqüentes em pacotes de química quântica, onde estas propriedades são obtidas através de métodos ab initio ou através da Teoria do Funcional de Densidade. As intensidades Raman estáticas são obtidas a partir de polarizabilidades geradas pela da ação de um campo elétrico de freqüência nula (estático), e, portanto são conhecidas como polarizabilidades estáticas. Quando as polarizabilidades tiverem sua origem associada à ação de um campo elétrico de freqüência não nula passam a ser chamadas de polarizabilidades dinâmicas, e as intensidades Raman provenientes destas polarizabilidades passam a ser designadas por intensidades Raman dinâmicas.Cálculos empregando polarizabilidades estáticas realizados por Fleisher e Pulay 2 para as moléculas de benzeno e coroneno em fase gasosa, no nível de teoria B3LYP/6-31G*, obtiveram uma concordância semiquantitativa para as intensidades Raman e razões de depolarização. Halls e Schlegel 3 , em um estudo comparativo envolvendo doze moléculas pequenas, utilizando os métodos ab initio Hartree-Fock, MP2 e vários funcionais (S-VWN, BLYP, B3LYP, MPW1-PW91), partindo de polarizabilidades estáticas, obtiveram seus melhores resultados no nível MP2. Entretanto, um fator comum a estes estudos baseados em polarizabilidades estáticas é que os mesmos não são capazes de reproduzir sequer o padrão de intensidades para uma particular freqüência de excitação, seja esta ressonante ou não, mesmo quando funções de base muito grandes como a augcc-pVTZ 3 são utilizadas. Uma solução parcial para este problema surgiu com Helgaker e colaboradores 4 , que desenvolveram uma metodologia e implementação computacional, nos níveis de teoria Hartree-Fock e MCSCF, para o cálculo de intensidades Raman dinâ-micas de moléculas em fase gasosa, a partir de polarizabilidades calculadas na mesma freqüência de excitação utilizada para obter o espectro (polarizabilidades dinâmicas). As polarizabilidades dinâmi-cas são obtidas através da Teoria da Resposta Linear (LRT) 5-7 e o gradiente da polarizabilid...

Recebido em 25/3/08; aceito em 6/5/08; publicado na web em 22/9/08 COMPUTATIONAL IMPLEMENTATION OF THE MODEL CHARGE-CHARGE FLUX-DIPOLE FLUX FOR CALCULATION AND ANALYSIS OF INFRARED INTENSITIES.The first computational implementation that automates the procedures involved in the calculation of infrared intensities using the charge-charge flux-dipole flux model is presented. The atomic charges and dipoles from the Quantum Theory of Atoms in Molecules (QTAIM) model was programmed for Morphy98, Gaussian98 and Gaussian03 programs outputs, but for the ChelpG parameters only the Gaussian programs are supported. Results of illustrative but new calculations for the water, ammonia and methane molecules at the MP2/6-311++G(3d,3p) theoretical level, using the ChelpG and QTAIM/Morphy charges and dipoles are presented. These results showed excellent agreement with analytical results obtained directly at the MP2/6-311++G(3d,3p) level of theory.Keywords: ab initio and DFT electronic structure methods; CCFDF/QTAIM and CCFDF/ChelpG absolute infrared intensities; molecular spectroscopy.