The density functional theory study on the 1,3-dipolar cycloaddition of carbon-methyl nitrone with acrlonitrile

摘要

Eight competing reaction pathways on the potential energy surface of C-methyl nitrone with acrlonitrile have been determined at the B3LYP/6-311++G~(**) level using the density functional theory, and each of them, which has a double-peak-double-valley structure, consists of reactants, a prior complex, two transition states and two products. Accurate geometric structures, relative stabilities and harmonic vibrational frequencies of the stationary points have been investigated. The changes of enthalpy, entropy and Gibbs free energy for each reaction path are also calculated. The Boltzmann equilibrium distributions for the eight puckered envelope conformations have also been discussed qualitatively through the calculations of Gibbs free energy at 20–1000 K. The absolute rate constant calculation at the room temperature has been performed using conventional transition state theory, and as a result the most probable reaction path is determined. The exothermal energies of the possible reaction paths range from 26.11 to 51.75 kJ/mol. The first potential barrier heights vary from 72.28 to 105.62 kJ/mol while the barrier heights of isomerization vary from 1.37 to 17.05 kJ/mol. It is found that the formation of H-bond plays a critical role in the reaction. The intramolecular H-bond increases the stability of the trans-C-methyl nitrone, and the intermolecular H-bond between the C-methyl nitrone and acrlonitrile results in the formation of the prior complex, both of which reduce the reactivity of the title system. Moreover, the relative energies calculated by higher level computations, including the MP2, MP3, MP4SDQ, and QCISD(T) methods, are consistent with that obtained from the B3LYP method, which indicates that the B3LYP/6-311++G~(**) level of theory can apply to this kind of systems.