THE MECHANISMS OF THE GAS-PHASE REACTIONS OF HO- WITH OXIRANE AND HS- WITH THIIRANE - AN ABINITIO STUDY

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(N)2 reaction of HO- with I has a negative activation barrier (-5.1 kcal mol-1) and produces HOCH2CH2O-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 -OCH2C(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 HSCH2CH2S- 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(N)2 transition state. Reaction profiles are presented and the theoretical studies are compared with relevant gas phase experiments. The proton affinities of -OCH2C(O)H and -SCH2C(S)H are calculated to be 358.8 and 334.6 kcal mol-1 respectively.