CASPT2//CASSCF Study on the Photolysis Mechanism of 2,3-Diazabicyclo[2.1.1]hex-2-ene: α C−N versus β C−C Cleavage

Abstract

A CASPT2//CASSCF study has been carried out to investigate the mechanism of the photolysis of 2,3-diazabicyclo[2.1.1]hex-2-ene under direct irradiation and triplet-sensitized irradiation. By exploring the detailed potential energy surfaces and surface crossing points for several low-lying excited states, we have been able to provide a tentative description on the photophysical and photochemical processes of this compound. According to our calculations, on the S(1) surface one C-N bond is broken first to generate the diazenyl biradicals, which then result in the photoproduct bicyclobutane through a concerted C-N cleavage and C-C coupling process. The intersystem crossing (ISC) from the S(1) state to the triplet state was found to be quite efficient, which could account for two other photoproducts (butadiene and 1,2-diazabicyclo[3.1.0]hex-2-ene) on direct irradiation. On the T(1) surface, the C-C bond cleavage could readily take place to generate the triplet hydrazonyl biradical. One deactivation path of this intermediate is to convert into the corresponding singlet biradical by an ISC process. Then this biradical undergoes a barrierless C-N bond formation to form the photoproduct 1,2-diazabicyclo[3.1.0]hex-2-ene. Another path of the triplet hydrazonyl biradical is to form the intermediate 2-allyl-diazomethane through a C-N cleavage. This intermediate will break another C-N bond to produce a terminal carbene species, which is easily converted into butadiene and bicyclobutane by 1,2-hydrogen shift reaction and addition to C=C double bond, respectively.