Theoretical study of the electrostatic and steric effects on the spectroscopic characteristics of the metal-ligand unit of heme proteins. 3. Vibrational properties of Fe(III)CN-.

The vibronic theory of chemical activation and quantum chemical calculations are applied to calculate the stretching vibrational frequency of cyanide, coordinated by the complex of ferric porphyrin with imidazole. The results show that the frequency of the stretching vibration of the cyanide strongly depends on its coordination geometry and is hardly affected by the electrostatic perturbations of reasonable magnitude. The comparison of these results with the experimental data on the cyanide complexes of different heme proteins and their models allows to elucidate the cyanide coordination geometry. The combined infrared and resonance Raman scattering experimental investigation of the cyanide and carbonyl complexes with the same heme protein is proposed to distinguish between the steric and electrostatic contributions to the heme-protein interaction.