We report kinetic and mechanistic studies of nitrogen elimination from 1- pyrazoline and its deuterated derivatives. This study was carried out for the gas phase employing 6-311þG(d,p) basis sets with B3LYP, MPW1PW91 and PBEPBE methods. This reaction may proceed through two different mechanisms, molecular and radical. The molecular mechanism advances via two different paths and corresponding intermediates. There are either a four-centred cyclic or a sixcentred cyclic intermediate as transition states (TSs) in the molecular mechanism. The calculated kinetic parameters and isotope effects for the molecular mechanism are consistent with experimental results for cyclopropane formation. However, from the kinetic isotope effects point of view, the fit is better for the radical mechanism for propene formation. Considering activation energies, theoretical calculations suggest that the radical mechanism is the less suitable. The B3LYP method results are in a better agreement with the experimental results when considering only computational data.

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