Potential energy surfaces for protonation of hydrochlorofluoromethanes

The proton affinities (PAs) and potential energy surfaces (PESs) of hydrochlorofluoromethanes (HCFMs) have been predicted by using Gaussian-3X (G3X) method. The G3X PAs agree with previous G3 predictions, while the large discrepancies between theoretical and experimental PAs persisted for CH2F2, CHF3, and CF3Cl. Protonated HCFMs usually have multiple structures, and structures with protonations at F-atom, [Methyl-FH](+), are the most stable. Transition states connecting different cation structures have been identified as proton exchanges between C-H sigma-bonds or between halogen atoms. While the high transition barriers hinder the isomerization between different cation structures, protonated HCFMs from proton transfer reactions of HCFMs with HCFM+/HCO+/HN2+ may have high enough energy to decompose to methyl(+) + HF/HCl or isomerize to less stable structures. Under low collision energy condition. reactions of methyl+ and H-2/HF/HCl form ion complexes only, except for CH3+ + HCl, where CH2Cl+ + H-2 can be formed by crossing over the transition barrier from [CH3-ClH](+) to [CH2Cl-H-2](+). (C) 2009 Elsevier B.V. All rights reserved.