Simulation of Non-Condon Enhancement and Interference Effects in the Resonance Raman Intensities of Metalloporphyrins

Abstract

The room-temperature resonance Raman (RR) spectra of nickel(II) porphine have been simulated for excitation within the Q-band optical transition, taking into account both coordinate-independent (Condon) as well as coordinate-dependent (non-Condon) contributions to the Raman scattering amplitude of each vibration. Computation of scattering intensities for all three classes of Raman-active modes (polarized, depolarized, and inversely polarized) observed in metalloporphyrin spectra was made possible in this fashion. Vibronic parameters were evaluated at the INDO level from the dependence of electronic transition moments and transition energies upon nuclear coordinates; the transform theory of RR scattering was then applied to compute the relative intensities of vibrational modes. The calculated enhancement patterns are in excellent agreement with experimental results. Three vibrational modes of b1g symmetry (ν10, ν11, and ν16) are identified as being the primary Jahn−Teller distortion coordinates: a substantial fraction of the total internal-mode reorganization energy for S0 − S1 photoexcitation is predicted to arise from symmetry-lowering. Interference between Condon and non-Condon scattering amplitudes is found to be a major determinant of the intensities of several polarized vibrations as well as those depolarized modes which are Jahn−Teller active. The dependence of scattering intensity on excitation wavelength for such modes is correctly predicted, indicating that the relative amplitudes and phasing of Condon and non-Condon contributions for each vibration are reliably determined.