DFT study of structure and vibrations in low-lying spin states of five-coordinated deoxyheme model

The results of the DFT (B3LYP) calculations of the structures and force fields of the five spin states of a model deoxyheme (C-s symmetry) are presented. It is shown that axial binding of imidazole dramatically reduces the gap between the intermediate- and high-spin states, so that energy of less than I kcal/mol is needed to push quintet states below the triplets. The calculated structures are in a good agreement with the known structures of various deoxy models of the heme,groups in myoglobin. A detailed analysis of low-frequency vibrations and their isotope shifts is given and compared with the experimental resonance Raman spectra, High-frequency CC and CN stretching modes show the observed lowering of their frequency with the increase of the core size of the molecule.