Semiempirical potential surfaces for the alkali hydrogen-halide reactions

Potential energy surfaces for 14 of the M+HX→MX+H reactions, (M=Li,Na,K,Rb; X=F,Cl,Br,I), were generated by a new VB semiempirical method. The dependence of barrier location and height on the bond angle were examined. The minimal barriers (=transition states) were found to occur in the bent configurations for the X=F reactions and in the collinear, or near-collinear, geometries for the X=Cl, Br, I reactions. This effect was accompanied by a shift of the barrier from the exit to the entrance channel and was shown to arise from a change in the effective HX distance at which an electron jump may be said to occur. A general tendency of increase in barrier height with increase in alkali atomic number was found. Relevant experimental observations including the energy disposal in the Ba+HX reactions, the role of vibrational excitations in the K+HCl, K+HBr reactions, the shape of the differential cross sections in the H+MX reactions and possible implications to the K+HBr, K+DBr, and K+TBr reactions are discussed.