On the fate of protonated chloroformates in the gas phase: a competition between forming HCl and chloroformic acid.

Abstract

Chloroformates are prevalent in the atmosphere due to their utilization as fuel additives and industrial solvents. These species may undergo interactions with atmospheric water resulting in protonated chloroformates. This study delves into the gas-phase dissociation of these protonated species. Tandem mass spectrometry was employed to scrutinize the unimolecular dissociation of protonated methyl (1), ethyl (2), neopentyl (3), and phenyl chloroformate (4). Notably, 1 and 4 exhibited HCl loss, yielding CH3OCO+ and C6H5OCO+, respectively, with 1 additionally generating neutral methanol and ClCO+. 4 additionally loses CO and CO2. In contrast, 2 and 3 each only exhibit a single fragmentation channel, with 2 losing C2H4 to generate protonated chloroformic acid and 3 generating protonated 2-methylbutene by losing neutral chloroformic acid. Density functional theory at the B3LYP/6-311+G(d,p) level of theory was employed to explore minimum energy reaction pathways for each ion, and CBS-QB3 single-point energy calculations were added to provide reliable energetics. The Rice-Ramsperger-Kassel-Marcus (RRKM) calculations of the rate constants for selected competing processes were carried out to link theory and experiment. One common unimolecular process observed was the 1,3-H shift of the proton from the carbonyl oxygen to the ester oxygen before dissociation.