Theoretical study of the thermal isomerization of isoxazole and 5-methylisoxazole

High-level ab initio and semi-empirical computations were carried out to investigate the mechanisms of thermal isomerization of isoxazole (1a) and 5-methylisoxazole (2a). The calculations were made over the whole isomerization potential energy surface at the MP2/cc-pVDZ and QCISD(T)/aug-cc-pVDZ//MP2/cc-pVDZ levels. The predicted rate-limiting transition states were refined further at the QCISD(T)/aug-cc-pVDZ//QCISD/cc-pVDZ level. Ab initio results are in excellent agreement with available experimental results. Furthermore, they allow us to propose a detailed reaction mechanism and to exclude reaction paths proposed earlier. We also report the results from semi-empirical computations. The MNDO method predicts reasonable values for the heat of formation of the reactants but it fails for other isomers and overestimates the activation energies for most reaction steps. The AM1 method yields improved values for the activation energies, at the expense of less reliable heats of formation for the minima in the potential energy surface. In general, both methods produce results that are good when the simplicity of the model is considered; however, there is no simple way to determine in advance when the results are meaningful. Thermochemical and kinetic parameters obtained by the different theoretical methods are discussed and compared with experimental values. Copyright (c) 2004 John Wiley & Sons, Ltd.