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

An MC-SCF/6-31G(*) study of the singlet and triplet Paterno-Buchi reaction (for the model system formaldehyde and ethylene) is presented. In addition to the computation of the relevant minima and transition structures, the Born-Oppenheimer violation regions, where a fast decay from the singlet excited state (S-1) to the ground state (S-0) surface takes place, have been fully characterized by locating and optimizing the structure of two different S-0/S-1 conical intersections. The photochemical mechanisms of oxetane formation via carbon-carbon (C-C) and carbon-oxygen (C-O) attacks have both been investigated. For the C-C attack 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 oxetane 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 intersection point at a C-O distance of 1.77 Angstrom before the diradical is fully formed. Thus, the system can evolve back to the reactant or produce a C-O bonded transient diradical intermediate that is isolated by very small barriers to fragmentation or ring-closure to oxetane. While the diradical structures corresponding to the two modes of attack differ 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 crossing from triplet to singlet will lead to the same diradical ground-state pathways that can be entered via singlet photochemistry.