Abstract
The kinetics of the photochemical (1,7)-sigmatropic shift of 1,3,5-cycloheptatriene (CHT) are determined by two-pulse, one-color, time-resolved UV resonance Raman spectroscopy. An amplified, synchronously pumped dye laser is frequency doubled, providing separate pump and probe pulses at 284 nm with 1.7-ps time resolution. The difference spectra reveal the presence of three scattering components: vibrationally relaxed, vibrationally unrelaxed, and excited-state CHT. Scattering from vibrationally relaxed CHT rises with a time constant of 26 {plus_minus} 10 ps. Scattering from vibrationally unrelaxed CHT appears with a time constant of 26 {plus_minus} 10 ps and decays with a time constant of 15 {plus_minus} 5 ps. Scattering from the excited state of CHT appears and decays within the time resolution of the instrument. The identical appearance kinetics of vibrationally relaxed and unrelaxed CHT demonstrates that population of the ground state followed by rapid intramolecular vibrational relaxation is complete in 26 ps, producing a Boltzmann population which vibrationally cools in 15 ps. These results show that the solution-phase photochemical sigmatropic shift in CHT is complete in 26 ps. 43 refs., 7 figs.
Published Version
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