RESONANCE RAMAN SPECTRA AND NORMAL MODE DESCRIPTIONS OF A BACTERIOCHLOROPHYLL a MODEL COMPLEX

Resonance Raman (RR) spectra are reported for the bacteriochlorophyll a (BChl a) model complex, copper methylbacteriopheophorbide a (CuBPheo a). The spectra are acquired at a number of excitation wavelengths in the Qy+ Qx and B regions of the absorption spectrum. The RR data obtained for CuBPheo a are correlated to those previously reported for BChl a. The normal modes of the BChl structure are calculated by using the semiempirical quantum chemistry force field (QCFF/ PI) method of Warshel and Karplus [J. Am. Chem. Soc.94,5612–5625 (1972)]. The vibrational data are used in conjunction with the calculations to obtain a cohesive assignment for the normal modes of BChl a with frequencies between 1100 and 1700 cm‐1. The forms of the normal modes of BChl a are predicted to be different both from those of chlorophyll a and from those of porphyrins.