Study of the Reactions of O- with CF4 and CHF3 by Ab Initio Calculations

Abstract

Abstract Previous experiments on the reactions of O- with CF4 and CHF3 using a selected ion flow tube (SIFT) have indicated that many species, such as CF3-, F-, and HF2-, are produced in the O- + CHF3 reaction, although no reaction proceeds in the O- + CF4 reaction under thermal conditions. Ab initio molecular orbital calculations were carried out in this study to clarify the reason for the difference in the reactivity between these two reactions. Potential energy surfaces (PESs) for the possible reaction channels were computed at the UMP2/6-31+G** level. Although the SN2-type reactions, such as the CF3O- + F channel, are exothermic, the calculated activation energy for the O- + CF4 reaction is rather high, 222 kJ/mol. The energies of the transition states in the O- + CHF3 reaction are below that of the reactant. We may conclude that the difference in reactivity between the two reactions is attributable to the charge distribution of each molecule. The H atom in CHF3 has a large effect on increasing the reactivity because the positively charged H atom attracts O-, whereas O- has difficulty approaching the CF4 molecule because of the four negatively charged F atoms.