Excited-state geometries derived from the analysis of resonance Raman spectra. Example: 1(π-π∗) state of 3,5-di-tert-butyl-o-benzoquinone

Abstract Resonance Raman spectra of 3,5-di-tert-butyl- o -benzoquinone were obtained by exciting its lowest 1 (π-π ∗ ) state. The resonance enhancement of two stretching modes (CC, CO) and of one CH bending mode was analyzed applying the Kramers-Heisenberg-Dirac relation. The mathematical fitting of the experimentally derived spectra yielded the magnitude of the Dushinsky mode mixing and the displacements of the potential curves along the considered normal modes. These displacements were transformed to bond-length changes with the use of the eigenvector matrix L of the ground-state vibrations. The excited-state geometry is characterized by the unsymmetric lengthening of the CC bonds within the acrolein subunits and a shortening of the CC bond inside the butadiene subsystem.