Biopolymers and cell. 2007; 23 (6): 519 - 528

Investigation of infrared spectrum of Fe(II) porphin in different spin states by quantum chemical density functional theory

B. F. Minaev, A. B. Minaev, D. N. Hovorun

The infrared (IR) absorption spectra of the Fe(II) porphin molecule (Fe(²²)P) are calculated by the quantum-chemical method of density functional theory (DFT) for the singlet, triplet, and quintet spin states. The UB3LYP functional with the 6-311G basis set is used in geometry optimization and IR calculations. The quintet state ⁵B_2g of the D_2h symmetry is found to be the ground state. Though the close-lying triplet ³À_2g and high-energy singlet ¹À_1g states belong to the D_4h symmetry, the IR spectra have been analyzed in terms of the lower symmetry D2h point group. All IR active vibrations are tabulated and discussed. The low-frequency modes with large out-of-plane displacements of Fe(II) ion have different IR intensities, normal vibrations, and frequency shifts in the quintet state in respect to the singlet and triplet states.

Keywords: Fe(II) porphin, singlet, triplet, quintet, spin states, density functional theory, IR absorption spectrum, low-frequency modes.