Chlorophyll model compounds: effects of low symmetry on the resonance Raman spectra and normal mode descriptions of nickel(II) dihydroporphyrins

Resonance Raman (RR) spectra are reported for a series of nickel(II) dihydroporphyrins with excitation in the B, Q/sub x/, and Q/sub y/ absorption regions. The molecules include trans-octaethylchlorin, ..gamma..,delta-deuteriated trans-octaethylchlorin, 9-deoxomethylmesopyropheophorbide a, methylmesopyropheophorbide a, and methylpyropheophorbide a. These molecules represent a series in which the structural complexities of chlorophyll a (reduced pyrrole ring, isocyclic ring, 9-keto group, and 2-vinyl group) are systematically added to the basic tetrapyrrole structure. All of the observed in-plane chlorin skeletal modes and vibrations of the isocyclic ring and vinyl group are assigned. Assignments are then proposed for chlorophyll a based on analogy to those of the Ni(II) complexes. In addition to the spectral assignments, normal coordinate calculations are performed on the various nickel(II) dihydroporphyrins. These calculations indicate that the forms of the normal coordinates of the metallochlorins bear little resemblance to those of the parent metalloporphyrins. In the low-symmetry environment which characterizes the reduced pyrrole pigments, a number of the vibrations are localized in semicircles or quadrants of the macrocycle rather than being delocalized over the entire ring. This vibrational localization is due to geometrical changes in the ..pi..-bonded system which occur as a result of reduction of one of the pyrrole rings and addition of the isocyclic ring.