THEORETICAL-MODEL AND ABINITIO CALCULATION OF POTENTIAL-ENERGY SURFACES FOR THE REACTION NH-PI)+H-2(1-SIGMA-G+)( .(2)

This paper examines the doublet potential energy surfaces (PESs) gover ning the NH+(H-2)-->NH2+(H) reaction by combining ab initio calculatio ns of the PES with a model Hamiltonian. It appears that in C2v ColliSi ons, the deep ground-state minimum is separated from the reactants by a barrier of about 1.2 eV. In the region around the barrier , several PESs exhibit crossings which become avoided for lower symmetry. Thus, in this region low energy paths between reactants and products can ari se which do not necessarily sample the deep minimum of the ground stat e. These paths, together with possible non-adiabatic transitions, can therefore account for the experimentally observed behaviour of the rea ction.