Pj. Reid et al., DETERMINATION OF PERICYCLIC PHOTOCHEMICAL-REACTION DYNAMICS WITH RESONANCE RAMAN-SPECTROSCOPY, Journal of physical chemistry, 98(22), 1994, pp. 5597-5606
Resonance Raman intensity analysis and picosecond time-resolved resona
nce 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-cyclo
heptatriene (CHT). The resonance Raman intensities of CHD demonstrate
that evolution along the conrotatory reaction coordinate occurs immedi
ately after photoexcitation, in agreement with the prediction of the W
oodward-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 inter
nal conversion to a lower energy, optically dark surface. The Raman in
tensities of COT and CHT demonstrate that for these molecules, the ini
tial 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 prerequ
isite 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 s
cale. Analysis of the time-resolved vibrational spectra also demonstra
tes that population of the ground state is followed by vibrational rel
axation 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.