Low-frequency vibrations of bacteriochlorophyll

Frequencies and normal vibrational modes of bacteriochlorophyll are calculated using the semiempirical quantum mechanical MNDO-PM3 method. To analyze the structure of normal modes, the indices of delocalization of the vibrations and the distribution functions of normal modes over atoms in the molecule are introduced. It is shown that normal vibrational modes of bacteriochlorophyll in the region from 3 to 20 cm(-1) represent "intermolecular" vibrational modes of phytol and tetrapyrrole macrocycle. As the vibrational frequency increases, the normal modes delocalized both on phytol and tetrapyrrole atoms alternate with the modes that are delocalized only on phytol atoms or only on tetrapyrrole atoms. The structural properties of some modes are considered in the aspect of their possible involvement in the formation of absorption spectra of the pigments of reaction centers in photosynthesis and in the formation in them of the coordinate of a primary reaction of the intermolecular electron transfer. (C) 2001 MAIK "Nauka/lnterperiodica".