DEUTERIUM-ISOTOPE EFFECT ON THE PHOTOISOMERIZATION RATES OF STILBENE - AN RRKM ANALYSIS BASED ON COMPUTED VIBRATIONAL FREQUENCIES

Microcanonical rate parameters k(E) for stilbene photoisomerization are obtained by RRKM calculation based on computed vibrational frequencies of reactant and transition state. Two alternative transition states are considered, one formed by the avoided crossing between the 1B and 2A energy curves and the other arising on the 1B energy curve; for the former, nonadiabatic effects are estimated by computing 1B-2A vibronic couplings. The thermal averaged rate parameters k(T) are obtained from the computed k(E). The comparison of the rate coefficients observed in supersonic beams and in the dense phases with the computed rates indicates that the photoisomerization of stilbene can be well described in the framework of a statistical theory and supports the nonadiabatic mechanism associated with the transition state formed by the 1B-2A avoided crossing. Both the rate parameters observed in supersonic beams and in the dense phase can be explained by attributing a different degree of adiabaticity and a different barrier height for the reaction in the two environments.