A THEORETICAL-STUDY OF PROPAGATION RATE COEFFICIENTS FOR METHACRYLONITRILE AND ACRYLONITRILE

The propagation rate coefficients for methacrylonitrile (MAN) and acry lonitrile (AN) were calculated using transition state theory and high- level ab initio molecular orbital theory. The calculations take partic ular 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 n ot perform very well), because of cancellations in the partition funct ion ratio in transition state theory; however, two of the internal rot ations studied were found to be sensitive to the chain length of the r adical used in the calculations. Activation energies were found to be extremely sensitive to the level of theory. At the highest level of th eory 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 acry lates. The higher frequency factors for methacrylates are largely due to hindrance caused by the methyl groups to the three transitional mod es in the transition state which correspond to the three external rota tional degrees of freedom of the monomer in the reactants (but not a r esult of increased hindrance to methyl rotation itself in the transiti on 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.