Depolarization dispersion curves of some Raman fundamentals in ferrocytochrome c and oxyhaemoglobin

Depolarization dispersion curves and Raman excitation profiles of three fundamentals in ferrocytochrome c (1585, 1312 and 1363 cm−1) and oxyhaemoglobin (1583, 1376 and 1638 cm−1) are calculated togther with the corresponding absorption spectra in the region of 16000‐23000 cm−1. A four‐orbital model with two‐electron interaction term and vibronic coupling terms for the a1g, a2g, b1g and b2g vibrations is used in two‐mode calculations. It is shown that the observed depolarization dispersion curves indicate the presence of symmetry‐breaking A2g and B1g electronic perturbations in ferrocytochrome c and of a B2g off‐diagonal electronic perturbation in oxyhaemoglobin. These electronic perturbations lower the D4h symmetry of the porphyrin ring. It is also demonstrated that smaller vibronic perturbations become active in these molecules at lower symmetry. These vibronic perturbations result mainly from the symmetry‐forbidden (D4h group) linear dependence of the nuclear potential energy in the Q and B states and of the two‐electron interaction term on non‐totally symmetric vibrational coordinates.