Unimolecular reactions in isolated and collisional systems: Deuterium isotope effect in the photoisomerization of stilbene

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Abstract
The isomerization of t-stilbene (stilbene h12 ) and three deuterated derivatives has been studied in a supersonic expansion, the thermal gas phase, and solution. In the jet we find that almost all effect of full deuteration (stilbene d12 ) is produced by deuteration of the two ethylinic hydrogens only (stilbene d2 ). Complete deuteration of the phenyl rings (stilbene d10 ) has rather little influence on the decay of the jet-cooled molecule. Nonexponential decays are found at intermediate excess energies in the jet-cooled system, with the degree of nonexponentiality decreasing with increasing excess energy. The ordering of the decay rates observed in the jet is not consistent with previous RRKM calculations of the isomerization rates of stilbene h12 and d2. Using similar parameters the calculations consistently place the stilbene d2 and stilbene d10 curves in the wrong order. Our results suggest extensive but not complete vibrational relaxation in the isolated molecule. Vibrational redistribution rapidly becomes complete in the presence of buffer gas. In thermal samples the isomerization rates of stilbene h12 and stilbene d10 are identical over a wide range of solvents and temperatures. By contrast the isomerization rates in stilbene d2 and stilbene d12 are 1.4 and 1.5 times slower than in stilbene h12. Again, these ratios appear constant over a wide range of experimental conditions.