Quantum chemical theory calculations on the mechanism of the homogeneous, unimolecular gas-phase elimination kinetics of selected diazirines

Theoretical calculations on the gas-phase thermal decomposition of dimethyldiazirine, diethyldiazirine and difluorodiazirine have been carried out using ab initio composite methods CBS-QB3 and G3, and DFT CAM-B3LYP, MPW1PW91, PBE1PBE and M062X. Reasonable agreement has been found with the experimental values by the G3 method. Two possible mechanisms were studied: Mechanism A consists the opening of the heterocyclic ring with hydrogen transfer to a nitrogen atom to form prop-1-en-2-yldiazene intermediate which later decomposes to nitrogen gas and the corresponding olefin. Mechanism B involves the extrusion of nitrogen molecule with the formation of carbene intermediate which subsequently yields the corresponding olefin. The results obtained from G3 calculations support the mechanism of a carbene intermediate. Population analysis and Wiberg's bond order show an interaction between the leaving nitrogen and the electron deficient carbon formed during the first step of the reaction. (C) 2015 Elsevier B.V. All rights reserved.