Qy-Excitation Resonance Raman Spectra of Chlorophyll a and

J. Phys. Chem. B 1997, 101, 9635-9644

9635

Qy-Excitation Resonance Raman Spectra of Chlorophyll a and Related Complexes. Normal Mode Characteristics of the Low-Frequency Vibrations Chengli Zhou, James R. Diers, and David F. Bocian* Department of Chemistry, UniVersity of California, RiVerside, California 92521-0403 ReceiVed: June 16, 1997; In Final Form: September 11, 1997X

Qy-excitation resonance Raman (RR) spectra are reported for film aggregates of chlorophyll (Chl) a and a series of related complexes. The latter include Mg(II) octaethylporphyrin (MgOEP), Mg(II) transoctaethylchlorin (MgOEC), methyl-9-desoxomesopyrochlorophyllide (DMPChl) a, and methylpyrochlorophyllide (MPChl) a. These complexes represent a series in which the structural complexities of Chl a (saturated pyrrole ring, isocyclic ring, C9-keto group, C2-vinyl group, and C10-carbomethoxy group) are systematically added to the basic tetrapyrrole architecture. On the basis of comparison of the RR scattering characteristics of the different complexes and the predictions of semiempirical normal coordinate calculations, a self-consistent set of vibrational assignments has been developed for all the RR active modes in the low-frequency regime (100-1000 cm-1). The studies indicate that the low-frequency vibrations encompass a diverse set of motions that include both the ring skeleton and peripheral substituent groups. However, modes in the very lowfrequency regime (