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 a 1g , a 2g , b 1g and b 2g vibrations is used in two‐mode calculations. It is shown that the observed depolarization dispersion curves indicate the presence of symmetry‐breaking A 2g and B 1g electronic perturbations in ferrocytochrome c and of a B 2g off‐diagonal electronic perturbation in oxyhaemoglobin. These electronic perturbations lower the D 4h 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 ( D 4h 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.