NORMAL-MODE CHARACTERISTICS OF CHLOROPHYLL MODELS - VIBRATIONAL ANALYSIS OF METALLOOCTAETHYLCHLORINES AND THEIR SELECTIVELY DEUTERATED ANALOGS

The resonance Raman (RR) and infrared (IR) spectra of the Zn, Cu, and Ni complexes of trans-octaethylchlorin (OEC) reveal significant differences in the vibrational-mode properties of metallochlorins and metalloporphyrins. Modes with a contribution from the C(a)C(m) stretching coordinate are distinguished by their sensitivity to metal substitution and to selective d2 and d4 methine deuteration. Comparison of the resonance Raman spectrum of CuOEC with that of CuECI (ECI = etiochlorin I) identifies those modes with a contribution from C(b)C(b) and C(b)C(a) stretching and C(b)C(s) bending coordinates. The results obtained show that there is substantial mixing of C(a)C(m) and C(b)C(b) stretching character in the high-frequency modes of MOEC. The suggestion21 that the symmetry reduction that occurs in metallochlorins relative to metalloporphyrins produces vibrations-mode localization to specific hemispheres or quandrants of the macrocycle has been tested and confirmed by specific d2 deuteration at the methine carbons. Resonance Raman spectra of CuOEP-d2 (OEP = octaethylporphyrin) and CuOEP-d4 establish that, for a delocalized mode, methine d2 deuteration can be expected to produce half the d4 shift. For CuOEC, selective deuteration at the alpha,beta and gamma,delta methine positions causes different patterns of frequency shifts that indicate the extent of mode localization. Analysis of these data shows that the normal modes of the MOEC class of compounds have varying degrees of localization; moreover, it identifies those modes that retain delocalized, porphyrin-like behavior, albeit with altered potential energy distributions relative to the MOEP situation. In agreement with the analysis by Boldt et al., however, we find that, in general, it is not possible to assign the vibrational modes of metallochlorins by direct analogy with metalloporphyrins. An exception to this occurs for several modes below 1350 cm-1 that are localized in the alpha,beta positions away from the reduced pyrole ring, but modes that are gamma,delta localized in this frequency region have no porphyrin counterparts.