A DFT study on the intramolecular dissociation pathways of ethyl fluoroformate radical cation in the gas phase; II. Keto path

The ground-state intramolecular gas-phase dissociation pathways of ethyl fluoroformate radical cation (FCOOCH2CH3·+) are studied using density functional molecular orbital methods. Initial geometries are optimized using the 6-31G* basis set. Electron correlation is incorporated by optimizing the geometries at the B3LYP level using the 6-31G** basis set. Stationary points are characterized by frequency calculation at the same level of theory and basis set. In the first installment of this paper, the existence of so-called enol pathway was reported and the dissociation mechanism was described in detail. In this installment, a new dissociation mechanism, a supposed keto pathway, is proposed. In this reaction channel, the ester linkage is immediately broken in a rate-determining E1 step producing FCOO and C2H5+ which reacts further in a bimolecular elimination mechanism to yield the same intermediates (FCOOH+C2H4⋅+) as the enol pathway. In a similar manner as the enol pathway, the keto pathway is terminated by a slow elimination of a proton from C2H4⋅+ by FCOOH which acts as a base in an E2 elimination scheme. The keto pathway is more accessible than the enol pathway explaining the relative heights of the MS peaks in the EI spectrum of ethyl fluoroformate.