Vibrational Analysis of a Strongly Correlated System, Pentamethine Streptocyanine Dye, Based on Observed Infrared and Raman Spectra and Density Functional Calculations

Abstract

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.

Vibrational Spectra and Structures of Long-Chain Streptocyanine Dyes: Effects of Electron−Vibration Interactions and Vibrational Polarizabilities

“…It is seen that the calculated vibrational wavenumbers (scaled uniformly by 0.9326, which is obtained by least-squares fitting) and relative IR intensities are in good agreement with the experimental results. It is especially noted that good agreement is obtained for the wavenumber of the strongly IR-active mode in the 1720-1500 cm -1 region, which is sensitive to the computational level as shown in Figure 5 of our previous study 9 for SC5OP. This result supports the validity of using the BHandHLYP functional for calculating the vibrational spectra of this series of molecules.…”

“…All the IR bands observed in KBr disk are significantly broader than those observed in solution, and the bands in the 1200-800 cm -1 region are congested and overlap significantly with perchlorate bands, as in the case of the IR spectrum of SC5. 9 However, the decrease in the wavenumber of the strong IR band in the 1800-1400 cm -1 region is clearly seen also in this figure.…”

“…The latter three bands are also considered to arise from the same vibrational modes as the strong IR bands, which are observed for this molecule at 1507, 1144, and 1098 cm -1 (Figure 3c), to which the contributions of the vibration along the bond-alternation coordinate are significant as shown in Table 2. While the Raman bands of the same character have only medium intensities in the case of SC5, 9 the 1536 cm -1 band of SC7 and the 1512 cm -1 band of SC9 are the strongest in their Raman spectra, in accord with the for S BA and g BA is the corresponding diagonal element of the G matrix.…”

“…In our previous study, 9 the IR and Raman spectra of a pentamethine streptocyanine dye [(5-dimethylaminopenta-2,4-dienylidene)ammonium perchlorate, alias SC5] have been observed in dimethyl sulfoxide (DMSO) solution and in the polycrystalline state, and vibrational analysis has been carried out by comparing the observed spectra with those calculated by the density functional method. The features observed in the IR and Raman spectra have been shown to be well-reproduced by the calculations, indicating that the vibrational force field, dipole derivatives, and polarizability derivatives obtained from the calculations are sufficiently reliable.…”

Kinetics and mechanism of the gas‐phase OH hydrogen abstraction reaction from methionine: A quantum mechanical approach

“…Full geometry optimizations were made for all the stationary points using the BHandHLYP hybrid HF‐density functionals and the 6‐311G(d,p) basis set. This functional was chosen on the basis of its proven effectiveness 38–49 and because unpublished results from our group have shown that B3LYP functional fails to properly describe H abstraction reactions from sulfur compounds. Zang et al 38 have shown that for proton transfer reactions in multiple hydrogen bond systems, only the hybrid BHandHLYP method (from a large set of DFT methods, including B3LYP) is capable of predicting the structure and energetic information of both the minimum energy and transition structures at a comparable accuracy with the MP2 level.…”