ACTIVATION BARRIERS FOR SERIES OF EXOTHERMIC HOMOLOGOUS REACTIONS .1. METAL ATOM REACTIONS WITH N2O

We recently observed that the activation barriers of O-atom abstraction reactions between metal atoms and N2O, in which both reactants are in their ground electronic states and the atoms contain no valence p electrons, vary systematically with the sums of the metal atom ionization potential and the energy required to promote a valence s electron to the lowest p orbital. It is shown here that this observation can be explained by the assumption that the activated complex results from the resonance interactions of ionic and covalent structures. Activation barriers for 43 reactions are calculated and where experimental measurements are available, are shown to be in good agreement with those. New interpretations are offered for literature data on the Ca and Cr reactions. The resonance treatment leads to a more general relationship in which activation barriers depend simultaneously on ionization potentials, electron affinities, promotion energies, and bond energies of the reactants. A number of further series of reactions, where activation barrier or rate coefficient trends with some of these parameters have been observed, are discussed and seen to be special cases of this formalism. Good agreement is found between the height of the barrier for the N2O + H --> N2 + OH reaction calculated by the present resonance treatment and an ab initio method.