RESONANCE RAMAN INTENSITY ANALYSIS OF THE PHOTOCHEMICAL HYDROGEN MIGRATION IN 1,3,5-CYCLOHEPTATRIENE

Absolute resonance Raman scattering excitation profiles are presented for 1,3,5-cycloheptatriene (CHT) excited in resonance with the 1A'-1A'' photochemically active electronic transition at 260 nm. The resonance Raman cross sections and the absorption spectrum are modeled to quantitate the geometric evolution occurring immediately after excitation. The vibrational scattering intensities are consistent with planarization of the CHT ring, while no evidence for evolution along the methylene CH stretch coordinate is observed. This suggests that the hydrogen-shift reaction proceeds sequentially with planarization of the ring occurring before any changes in CH bonding. Analysis of the absolute resonance Raman intensities, yielding an excited-state homogeneous line width of approximately 100 cm-1, as well as the low fluorescence quantum yield of CHT (1.4 x 10(-6)) shows that the excited-state population decay occurs on a 25-50-fs time scale. These results indicate that evolution along ring planarization coordinates leads to rapid internal conversion of the initially prepared excited state to a lower-energy surface (not previously considered in orbital symmetry models) upon which the sigmatropic shift occurs.