A theoretical study of hydroxycarbene as a model for the homolysis of oxy-and dioxycarbenes

Recent work has shown that dioxycarbenes can readily undergo homolytic frag mentations to radicals in solution. Previous workers have found some theore tical evidence for a transition state in the homolysis of dihydroxycarbene to HOCO and H radicals using the CISD method, but they were not able to con clude whether such a barrier was real or an artifact of the approximation. In this work the homolysis of hydroxycarbene is examined at the CAS and MRC I levels of theory using the cc-pVDZ basis set. The atomic and molecular pr operties are examined using the theory of atoms in molecules. At the highes t level of theory, a transition state is round for the fragmentation of tra ns- but not of cis-hydroxycarbene. This transition state is rationalized in terms of the electronic states involved in the avoided crossing and by exa mining the evolution of several atomic and molecular properties during the homolysis. it is concluded that its origin can be traced to a mismatch of t he electronic structures of these states in the region of the avoided cross ing, best expressed by the dipole moments.