To Be or Not To Be Symmetric: That Is the Question for Potentially Active Vibronic Modes

Electronic spectra often exhibit vibronic structure when vibrational and electronic transitions occur in concert. Theory reveals (1) that orbital symmetry considerations determine specific roles played by the nuclear degrees of freedom and (2) that the vibrational excitation is often highly regiospecific, that is, attributable to an identifiable subset of atoms within the molecule. Spectra obtained from a chromium­(III) complex involving a macrocyclic ligand and two axially disposed butadiynide groups nicely illustrate many of the concepts involved.