A semi-empirical method for the estimation of activation barriers and kinetic parameters of atom transfer reactions

Abstract

The activation barriers of atom transfer reactions AB + C → A + BC have been estimated using the empirical Morse functions of the bonds AB and BC and the structural parameters of the activated complex, obtained at various approximation levels. Further, the rate constants and activation energies of a series of hydrogen transfer reactions have been calculated using the transition state theory formalism, including an Eckart tunnelling correction, and ab initio or standard structural parameters and vibrational frequencies. In general, the results obtained are fairly satisfactory. Moreover, it was shown that the barrier heights of hydrogen abstraction reactions linearly depend on the corresponding energy changes. Thus, these processes appear to be essentially under thermodynamic control. Finally, the effects of substituents on the barrier heights of hydrogen abstraction reactions from methane derivatives were rationalized. It appears that, whatever the attacking species (C) may be, the lowest barriers correspond always to high stabilized carbon centred radicals (A) and (or) very destabilized molecules (AB).