Synchronous concerted multiple-body photodissociation of oxalyl chloride explored by ab initio-based dynamics simulations

摘要

Photo-induced multiple body dissociation is of fundamental interest in chemistry and physics. A description of the mechanism associated with n-body (n ≥ 3) photodissociation has proven to be an intriguing and yet challenging issue in the field of chemical dynamics. Oxalyl chloride, (ClCO)_2, is the sole molecule reported up to date that can undergo four-body dissociation following absorption of a single UV photon, with a rich history of mechanistic debate. In the present work, the combined electronic structure calculations and dynamics simulations have been performed at the advanced level, which provides convincing evidence for resolving the mechanistic debate. More importantly, synchronous and asynchronous concertedness were explored for the first time for the (ClCO)_2 photodissociation, which is based on the simulated time constants for the C-C and C-Cl bond fissions. Upon photoexcitation of (ClCO)_2 to the S1 state, the adiabatic C-C or C-Cl fission takes place with little possibility. The four-body dissociation to 2Cl(~2P) and 2CO(~1Σ) was determined to a dominant channel with its branch of ～0.7, while the three-body dissociation to ClCO(~2A′) + CO(~1Σ) + Cl(~2P) was predicted to play a minor role in the (ClCO)_2 photodissociation at 193 nm. Both the four-body and three-body dissociations are non-adiabatic processes, which proceed in a synchronous concerted way as a result of the S_1 → S_0 internal conversion. There is a little possibility for two-body dissociation to occur in the S_0 and S _1 states.