An ab initio study of the photochemical decomposition of 3,3-dimethyldiazirine

Photochemical decomposition of 3,3-dimethyldiazirine (DMD) has been computa tionally investigated by using high-level ab initio calculations in conjunc tion with the 6-31G* and cc-pvdz basis sets. The geometries of minima and t ransition states, as well as conical intersection points in the seam of cro ssing of two surfaces, have been optimized with the complete active space s elf-consistent field (CAS-SCF) method, and their energies, recalculated wit h second-order multireference perturbation (CAS/MP2) theory. The reaction p ath starting at the excited n-pi* state of DMD is predicted to occur via a nonadiabatic mechanism, giving carbene and molecular dinitrogen (both in th eir singlet ground states) as the main products; the computed barrier heigh t (1.0 kcal mol(-1)) agrees well with the experimental estimate of the acti vation energy in the singlet excited state (0.0-1.5 kcal mol(-1)). Ground s tate of dimethylcarbene is the only species where a 1,2-hydrogen shift take s place, being the only source of propene. The calculated potential energy barrier height for dimethylcarbene to propene isomerization (2.6 kcal mol(- 1)) agrees well with the observed activation energy (2.56 kcal mol(-1)). No evidence for rearrangement in the first singlet excited state of DMD has b een found; such a process would lead to a higher activation energy than the observed one. Consequently, 1,2-hydrogen migration concurrent with N-2 ext rusion in the excited state has been ruled out.