Photoisomerization Reactions of Cyclopropene and 1,3,3-Trimethylcyclopropene: A Theoretical Study.

The mechanisms of the photochemical isomerization reactions were investigated theoretically using two model systems, cyclopropene and 1,3,3-trimethylcyclopropene with the CASSCF/6-311G(d) (six-electron/six-orbital active space) and MP2-CAS/6-311G(d,p)/CASSCF/6-311G(d) methods. The structures of the conical intersections, which play a decisive role in such photorearrangements, were obtained. The intermediates and transition structures of the ground states were also calculated to assist in providing a qualitative explanation of the reaction pathways. Our model investigations suggest that the preferred reaction route for both cyclopropene and 1,3,3-trimethylcyclopropene is as follows: reactant → Franck-Condon region → local minimum → transition state → conical intersection → local intermediate → transition state → photoproduct. The theoretical findings suggest that the conical intersection mechanism found in this work gives a good explanation and supports the experimental observations. We also investigated the thermal (dark) reaction mechanisms for the hydrogen migration reactions. Again, all the relative yields of final products predicted based on the present work are in good agreement with the available experimental findings.