The mechanisms of the gas phase reactions of HO − with oxirane and HS − with thiirane. An ab initio study

Abstract

Ab initio calculations at the MP2/6-311+G*//HF/6-31+G* level are used to characterize the mechanisms of the gas phase reactions of HO − with oxirane ( I) and HS − with thiirane ( II). The S N2 reaction of HO − with I has a negative activation barrier (−5.1 kcal mol −1) and produces HOCH 2CH 20 − with 47.2 kcal mol −1 of excess internal energy. This ion is capable of an intramolecular proton transfer and can decompose by loss of H 2 to give −OCH 2C(O)H. The transition state for H 2 loss is 17 kcal mol −1 more stable than the reactants. The ring-opening reaction of HS − + thiirane has a small barrier (+ 0.6 kcal mol −1) and produces HSCH 2CH 2S − with 23.5 kcal mol −1 of excess internal energy. There is virtually no barrier to intramolecular proton transfer within this thiolate; however, there is a large barrier to H 2 loss — the transition state is 18.4 kcal mol −1 less stable than the reactants. In both systems, the effects of ring strain relief are almost fully realized in the S N2 transition state. Reaction profiles are presented and the theoretical studies are compared with relevant gas phase experiments. The proton affinities of −OCH 2C(O)H and −SCH 2C(S)H are calculated to be 358.8 and 334.6 kcal mol −1 respectively.