Channels to singlet and triplet phenylcarbenes in phenyldiazomethane: a CASSCF and MRCI study

Diazo compounds such as phenyldiazomethane (C(6)H(5)C(H)N(2)) exhibit intriguing phenomena including the ultrafast formation of singlet carbene and the excited-state rearrangement reaction (RIES). In this work, we have used multi-reference configuration interaction with single and double excitations (MRCI-SD) and complete active space self-consistent field (CASSCF) methods to study the photodissociation dynamics of C(6)H(5)C(H)N(2). The equilibrium structures, transition states in the lowest three electronic states (S(1), T(1), and S(0)), and S(1)/S(0) and T(1)/S(0) minimum-energy crossing points both in the Franck-Condon region and on the pathway of the CN bond dissociation have been optimized. On the basis of the calculated S(1), T(1), and S(0) potential energy surfaces, we have uncovered the most efficient pathways to the lowest singlet and triplet phenylcarbenes (C(6)H(5)CH) in irradiated C(6)H(5)C(H)N(2).