title: A computational study of the mechanisms of the photoisomerization reactions of monocyclic and bicyclic olefins.
abstract: The mechanisms of the photochemical isomerization reactions were investigated theoretically using a model system of cyclohexene (1), cycloheptene (2), norbornene (3), and two bicyclic olefins (4 and 5) using the CASSCF (six-electron/six-orbital active space) and MP2-CAS methods with the 6-311(d,p) basis set. The structures of the conical intersections, which play a decisive role in such photoisomerizations, were obtained. The intermediates and transition structures of the ground state were also calculated to assist in providing a qualitative explanation of the reaction pathways. Two photoreaction pathways were examined in the present work. The first can produce a photoproduct with an extra ring. The other can yield a photoproduct with a smaller ring with an external double bond. Both pathways involve cyclic carbene intermediates. Also, our model investigations suggest that both reaction pathways follow a similar photochemical pattern as follows: reactant → Franck-Condon region → conical intersection → cyclic carbene intermediate → transition state → photoproduct. Moreover, these two reaction pathways can compete with each other since the energetics of their conical intersection points are quite similar. Our present theoretical results agree with the available experimental observations.
authors.affiliation: Department of Applied Chemistry, National Chiayi University, Chiayi, Taiwan. midesu@mail.ncyu.edu.tw
authors.forename: Ming-Der
authors.initials: MD
authors.lastname: Su
mesh_terms: D001643:Bridged Bicyclo Compounds / Q000737:chemistry*; D053138:Cyclohexenes; D003516:Cycloparaffins / Q000737:chemistry*; D015394:Molecular Structure; D010777:Photochemistry; D011789:Quantum Theory; D014466:Ultraviolet Rays
chemical_list: D001643:Bridged Bicyclo Compounds; D053138:Cyclohexenes; D003516:Cycloparaffins; C052568:cyclohexene