Theoretical Studies of Eliminations. 5. Intermolecular vs Intramolecular Eliminations:  An ab Initio Study of the Gas-Phase Reaction of NH2 - with CH3CH2SCH3

Ab initio calculations at the MP2/6-31+G**//MP2/6-31+G* level have been completed on the title reaction. The computational results are in good accord with a mechanism suggested by Nibbering and co-workers in 1987. The preferred elimination pathway involves abstraction of an α-proton followed by an intramolecular syn elimination. Although a conventional E2 mechanism is viable, abstraction of an α-proton has a substantial energetic advantage and is effectively irreversible. The intramolecular syn elimination has a higher barrier than a conventional anti E2 pathway, but is the only option after the irreversible α-proton abstraction. Results are also presented for the reaction of fluoride with CH3CH2SCH3.