Investigation of the frequency-dispersion effects on the Raman spectra of small polyenes

The time-dependent Hartree-Fock scheme is applied for determining the frequency-dependent Raman intensities of C_2nH_2n+2 molecules with n = 1-3. This analytic scheme, recently developed and implemented in the GAMESS program (Quinet, O.; Champagne, B. J Chem Phys 2001, 115, 6293), takes advantage of the 2n + 1 rule to express the polarizability derivatives in terms of first-order derivatives. It is found that including frequency dispersion strongly modifies the intensity activity coefficients of many vibrational normal modes and therefore changes the aspect of the spectra. On the other hand, the depolarization ratio is much less frequency dependent.