We have employed Rayleigh−Schrödinger perturbation theory to obtain a simple quantum mechanical model to simulate the influence of asymmetric distortions on the optical spectra of metalporphyrins. The model is based on Gouterman's classical four orbital model and considers group theoretical restriction for the interactions between different electronic states. In heme proteins, asymmetric distortions of the porphyrin macrocycle are induced by axial ligands, asymmetric peripheral substituents, and the anisotropic protein environment. They give rise to electronic and vibronic perturbations, which cause additional mixing between vibronic states and, for B 1 g -type perturbations, also, a split of the Q and B states, which are 2-fold degenerated in symmetric porphyrins exhibiting a D 4 h symmetry. To compare our calculations with experimentally observed splits of the Q band in the spectra of various ferrocytochrome c (Manas et al. J. Phys. Chem. B. 1999, 103, 6344) we estimated vibronic matrix elements from resonance Raman data of horse heart cytochrome c and some metalloporphyrins in solution. We investigated different combinations of electronic and vibronic perturbations to show evidence that both contribute equally to the observed Q-band split. Moreover, we found that vibronic perturbations give rise to different Q- and B-band splitting, in agreement with what was experimentally observed for asymmetrically distorted hemes and porphyrins. Our theoretical insights do not support the notion that a uniform electric field causes a splitting of bands in the optical spectrum of porphyrins.

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