Vibronic interaction in metalloporphyrin π-anion radicals

The vibronic (vibrational-electronic) interactions in the π-anion radicals of the metalloporphyrins (M=Cr, Mn, Fe, Co, Ni, Cu, and Zn), which show delocalized D_4h structures in the neutral states, are discussed using B3LYP density-functional-theory calculations. The B_1g and B_2g modes of vibration can remove the degenerate ²E _g state of the π-anion radicals in the D_4h symmetric structures to lead to rectangular and diamond D_2h distortions, respectively. Calculated vibronic coupling constants demonstrate that the Big modes of vibration better couple with the degenerate electronic state, leading to the rectangular D₂h distortion. In particular, the Big modes of v₁₀ and v₁₁, which have dominant contributions from C _α-C_m and C_β-C_β stretching, give large vibronic coupling constants in the π-anion radicals. The vibronic coupling constant can be viewed as the Jahn-Teller distortion force, and therefore these C-C stretching B_1g modes will play a central role in the Jahn-Teller effect of the π-anion radicals of the metalloporphyrins.