The Raman and infrared bands of trans-stilbene in the ground electronic state (S0) and an excited singlet
state (S1) are assigned on the basis of density functional calculations at the 6-311+G** level for the S0 state
and configuration interaction single calculations with the 6-311+G** basis set for the S1 state. Not only the
wavenumbers of normal modes but also Raman activities, Raman depolarization ratios, and infrared intensities
are calculated and used for band assignments. The vibrational patterns of some characteristic modes are
discussed. It is found that, with respect to a few low-wavenumber modes in both the S0 and S1 states, the
results of the present calculations are inconsistent with those derived from an analysis of the fluorescence
excitation spectra and dispersed fluorescence spectra of trans-stilbene in a supersonic jet.
Vibrational analysis is carried out for the organic (cationic) part of a pentamethine streptocyanine dye,
[(CH3)2N(CH)5N(CH3)2]+ClO4
- (alias SC5), by measuring its infrared and Raman spectra in solution and in
the polycrystalline state and by calculating the vibrational force field and the IR and Raman intensities by the
ab initio molecular orbital and density functional methods. It is found that a reasonable set of structural
parameters and vibrational force field can be obtained for the SC5 organic part at the BHandHLYP/6-31G*
level. The observed features of the IR and Raman spectra, including relative intensities, are well reproduced
by the calculations at this theoretical level. Two strong IR bands observed in the 1600−1200-cm-1 region
arise from the delocalized b1 modes along the bond-alternation coordinate of the conjugated chain. The
strong IR intensities are explained by large charge fluxes induced by these modes due to the strong electron−vibration interaction. These modes also appear in the Raman spectrum in solution because of the interaction
with the perchlorate ion existing at an asymmetric position near the conjugated chain. A delocalized a1
mode of the conjugated chain gives rise to a strong Raman band. Examination of the IR and Raman intensities
and the vibrational force constants clearly shows that the conjugated chain of the SC5 organic part is a strongly
correlated system. A detailed analysis of the origin of the IR and Raman intensities shows that the potential
energy distribution is not necessarily a good indicator of the origin of intensities.
Raman intensities of a charged conjugated π-electron system induced by electrostatic intermolecular interaction are studied theoretically. By using a simple Hamiltonian based on a two-state model, in which the response of the system to an electric field is taken into account, the formulas for the polarizability derivative and related quantities are derived. These formulas are applied to the case of a pentamethine streptocyanine dye, which has a symmetric conjugated chain consisting of four CC bonds with one NC bond on each end. It is shown that a reasonable magnitude of electrostatic interaction with a counterion induces Raman intensities on the order of 10 2 Å 4 amu -1 for the modes with large contributions from the vibration along the bondalternation coordinate of the chain, explaining the appearance of the 1574-and 1207-cm -1 bands in the Raman spectrum measured in solution. A slight deformation of the conjugated chain along the bond-alternation coordinate induced by electrostatic interaction and the strong electron-vibration interaction are responsible for these Raman intensities. The formulas derived in this study are then used for evaluating the electronic and vibrational contributions to the first hyperpolarizability. It is concluded that the vibrational contribution is on the same order of magnitude as (but smaller than) the electronic one, at least in the case of typical charged conjugated π-electron systems.
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