A theoretical study of propagation rate coefficients for methacrylonitrile and acrylonitrile

Abstract

The propagation rate coefficients for methacrylonitrile (MAN) and acrylonitrile (AN) were calculated using transition state theory and high-level ab initio molecular orbital theory. The calculations take particular account of internal rotations in the transition states. Frequency factors and rotational potentials were found to be insensitive to the level of theory used (except that the semiempirical AM1 method does not perform very well), because of cancellations in the partition function ratio in transition state theory; however, two of the internal rotations studied were found to be sensitive to the chain length of the radical used in the calculations. Activation energies were found to be extremely sensitive to the level of theory. At the highest level of theory used, the calculated frequency factor for MAN was slightly lower than experiment, while the activation energy was 2.6 kJ mol⁻¹ higher than experiment. Theoretical comparison of propagation of MAN and AN was used to explain differences observed experimentally in activation energies and frequency factors of methacrylates and corresponding acrylates. The higher frequency factors for methacrylates are largely due to hindrance caused by the methyl groups to the three transitional modes in the transition state which correspond to the three external rotational degrees of freedom of the monomer in the reactants (but not a result of increased hindrance to methyl rotation itself in the transition state). The higher activation energies of methacrylates arises from differences in hindrance and loss of delocalization in the transition states of the methyl-substituted and unsubstituted monomers.