Spin-orbit induced radiationless transitions in organometallics: Quantum simulation of the intersystem crossing processes in the photodissociation of HCo(CO)4

A theoretical description of the “fast” (<50 ps) intersystem crossing (ISC) processes occurring during the photodissociation of HCo(CO)4 is presented. The radiationless transitions are simulated by wave packet propagations on spin-orbit coupled two-dimensional potential energy surfaces (CASSCF/CCI) calculated along two reaction coordinates (qa=[Co–H] and qb=[Co–COax]). A mechanism of deactivation of the singlet excited state of HCo(CO)4 initially populated on UV excitation has been proposed. This mechanism differs slightly from the one deduced from a one-dimensional simulation performed separately, either along the Co–H bond or along the Co–COax bond: (i) in a very short time scale (<20 fs) 35% of the system dissociates toward the primary products H+Co(CO)4 (1E), whereas the 1E→3A1 and 1E→3E intersystem crossings occur within 50 ps; (ii) as soon as the lowest triplet states are populated, the system dissociates either to H+Co(CO)4 (3A1) or to H+Co(CO)4 (3E) on the corresponding potential energy surfaces; (iii) the indirect photodissociation mechanism through ISC may be described as a succession of elementary transitions followed by the homolysis of the metal–hydrogen bond.