Resonance Raman spectra of metalloporphyrins. Effects of Jahnndash;Teller instability and nuclear distortion on excitation profiles of Stokes fundamentals

摘要

Excitation profiles of depolarized modes (blg,b2g) in Ni etioporphyrin I exhibit marked 0ndash;0 resonance Raman intensity uponQhyphen;band excitation. By contrast, inversely polarized Raman scattering (a2g) is stronger with 0ndash;1 excitation. Within the crude Bornndash;Oppenheimer approximation, a theoretical model is presented which explains the 0ndash;0 enhancements of depolarized modes as arising from interference of intermanifold (Qthinsp;ndash;B) and intramanifold (Qthinsp;ndash;Q) vibronic coupling. Comparison of computed and observed excitation profiles shows that a weak Jahnndash;Teller distortion is present in theQstate. The vibronic model predicts behavior of Raman intensity in theB(Soret) electronic state. Evidence of increased vibronic coupling of both 400 and 1365 cmminus;1polarized vibrations (a1g) is found in chromium tetraphenylporphyrin with the appearance of a supernumerary peak at 1400 cmminus;1in the excitation profile of the 400 cmminus;1vibration. Excitation profiles ofa2gmodes in the chromium complex exhibit an apparent decrease in excited state vibrational frequencies due to increased coupling. Estimates of vibronic coupling strengths for theQstate are given for the nickel and chromium porphyrins.