THE DETERMINATION OF EXCITED-STATE GEOMETRIES BY ANALYZING RESONANCE RAMAN-SPECTRA - EXAMPLE (1)(PI-PI-ASTERISK) STATE OF O-CHLORANIL

Resonance Raman spectra of o-chloranil were obtained by exciting the e nergetically lowest pi-pi transition. Raman signals of three totally symmetric stretching modes (C=C, C=O, and C-C) showed pronounced inten sity alterations. The intensity changes of these modes were analyzed u sing a dispersion formula which is based on the Kramers-Heisenberg-Dir ac equation. Three-dimensional Franck-Condon integrals were calculated with the algorithm of Kupka and Cribb. The mathematical fitting of th e experimentally derived spectra yielded the displacements of the pote ntial curves along the considered normal coordinates and data of the D uschinsky mode mixing. The geometrical shifts along directions of grou nd-state normal coordinates were transformed to bond-length changes wi th the use of the eigenvector matrix L of the ground-state vibrations. L was obtained by a complete force field analysis. The excited-state geometry of o-chloranil is characterized by the contraction of the di- keto system accompanied by a weakening of the acrolein unit. The large st changes appear in the butadiene subsystem, where a conversion of bo nd order occurs between the double bonds and the single bond.