AN MC-SCF STUDY OF THE (PHOTOCHEMICAL) PATERNO-BUCHI REACTION

An MC-SCF/6-31G() study of the singlet and triplet Paterno-Buchi reac tion (for the model system formaldehyde and ethylene) is presented. In addition to the computation of the relevant minima and transition str uctures, the Born-Oppenheimer violation regions, where a fast decay fr om the singlet excited state (S-1) to the ground state (S-0) surface t akes place, have been fully characterized by locating and optimizing t he structure of two different S-0/S-1 conical intersections. The photo chemical mechanisms of oxetane formation via carbon-carbon (C-C) and c arbon-oxygen (C-O) attacks have both been investigated. For the C-C at tack the singlet mechanism can be concerted as the decay to the ground state takes place in a point where the C-C bond is fully formed. Thus , starting from this decay point, the system can evolve directly to ox etane or produce a C-C bonded transient diradical intermediate. The C- O attack leads to a nonconcerted path only. In this case, the excited- state branch of the reaction coordinate terminates in a conical inters ection point at a C-O distance of 1.77 Angstrom before the diradical i s fully formed. Thus, the system can evolve back to the reactant or pr oduce a C-O bonded transient diradical intermediate that is isolated b y very small barriers to fragmentation or ring-closure to oxetane. Whi le the diradical structures corresponding to the two modes of attack d iffer in energy by only 8 kcal mol(-1), the S-1 to S-0 decay point for C-C attack lies 33 kcal mol(-1) below the corresponding point for C-O attack. The triplet diradicals have energies and geometries that are very similar to the singlets. Thus we predict that intersystem crossin g from triplet to singlet will lead to the same diradical ground-state pathways that can be entered via singlet photochemistry.