Insights into mechanistic photodissociation of acetyl chloride by ab initio calculations and molecular dynamics Simulations

摘要

The potential energy surfaces of dissociation and elimination reactions for CH3COCl in the ground (S-0) and first excited singlet (S-1) states have been mapped with the different ab inito calculations. Mechanistic photodissociation Of CH3COCl has been characterized through the intrinsic reaction coordinate and ab initio molecular dynamics calculations. The (alpha-C-C bond cleavage along the S, pathway leads to the fragments of COCl((2)A") and CH3 ((2)A') in an excited electronic state and a high barrier exists on the pathway. This channel is inaccessible in energy upon photoexcitation of the CH3COCl molecules at 236 nm. The S-1 alpha-C-Cl bond cleavage yields the Cl(P-2) and CH3CO((X) over tilde (2)A ') fragments in the ground state and there is very small or no barrier on the pathway. The S-1 alpha-C-Cl bond cleavage proceeds in a time scale of picosecond in the gas phase, followed by CH3CO decomposition to CH3 and CO. The barrier to the C-Cl bond cleavage on the S-1 surface is significantly increased by effects of the argon matrix. The S-1 alpha-C-Cl bond cleavage in the argon matrix becomes inaccessible in energy upon photoexcitation of CH3COCl at 266 nm. In this case, the excited CH3COCl(S-1) molecules cannot undergo the C-Cl bond cleavage in a short period. The internal conversion from S-1 to S-0 becomes the dominant process for the CH3COCl(S-1) molecules in the condensed phase. As a result, the direct HCl elimination in the ground state becomes the exclusive channel upon 266 nm photodissociation of CH3COCl in the argon matrix at 11 K.