VIBRATIONAL ANALYSIS OF A STRONGLY CORRELATED SYSTEM, PENTAMETHINE STREPTOCYANINE DYE, BASED ON OBSERVED INFRARED AND RAMAN-SPECTRA AND DENSITY-FUNCTIONAL CALCULATIONS

Vibrational analysis is carried out for the organic (cationic) part of a pentamethine streptocyanine dye, [(CH3)(2)N(CH)(5)N(CH3)(2)]+ClO4- (alias SC5), by measuring its infrared and Raman spectra in solution a nd in the polycrystalline state and by calculating the vibrational for ce field and the IR and Raman intensities by the ab initio molecular o rbital and density functional methods. It is found that a reasonable s et of structural parameters and vibrational force field can be obtaine d 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. Tw o strong IR bands observed in the 1600-1200-cm(-1) region arise from t he delocalized b(1) modes along the bond-alternation coordinate of the conjugated chain. The strong IR intensities are explained by large ch arge fluxes induced by these modes due to the strong electron-vibratio n interaction. These modes also appear in the Raman spectrum in soluti on because of the interaction with the perchlorate ion existing at an asymmetric position near the conjugated chain. A delocalized al mode o f the conjugated chain gives rise to a strong Raman band. Examination of the LR and Raman intensities and the vibrational force constants cl early shows that the conjugated chain of the SC5 organic part is a str ongly correlated system. A detailed analysis of the origin of the IR a nd Raman intensities shows that the potential energy distribution is n ot necessarily a good indicator of the origin of intensities.