DETERMINATION OF PERICYCLIC PHOTOCHEMICAL-REACTION DYNAMICS WITH RESONANCE RAMAN-SPECTROSCOPY

Resonance Raman intensity analysis and picosecond time-resolved resonance Raman spectroscopy are used to elucidate the reaction dynamics of the electrocyclic ring-openings of 1,3-cyclohexadiene (CHD) and 1,3,5-cyclooctatriene (COT) as well as the hydrogen migration in 1,3,5-cycloheptatriene (CHT). The resonance Raman intensities of CHD demonstrate that evolution along the conrotatory reaction coordinate occurs immediately after photoexcitation, in agreement with the prediction of the Woodward-Hoffmann rules. The 900-cm(-1) optical T-2 combined with the 2 X 10(-6) fluorescence quantum yield shows that the initially prepared excited state of CHD depopulates on the 10-fs time scale due to internal conversion to a lower energy, optically dark surface. The Raman intensities of COT and CHT demonstrate that for these molecules, the initial excited-state dynamics consist principally of ring planarization with no evidence for motion along reactive coordinates. This suggests that the establishment of a planar excited-state geometry is a prerequisite for reactive pericyclic nuclear motion. Picosecond time-resolved resonance Raman Stokes and anti-Stokes spectra of the above reactions reveal that the ground-state photoproducts appear on the 10-ps time scale. Analysis of the time-resolved vibrational spectra also demonstrates that population of the ground state is followed by vibrational relaxation and single-bond isomerization of the ring-opened photoproducts on the 10-ps time scale. This work demonstrates that resonance Raman spectroscopy is a powerful methodology for elucidating condensed-phase chemical reaction dynamics.