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

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