  We investigate the potential energy surfaces and activation energies for
reactions between methyl halide molecules CH$_{3}X$ ($X$ = F, Cl, Br, I) and
alkali-metal atoms $A$ ($A$ = Li, Na, K, Rb) using high-level {\it ab initio}
calculations. We examine the anisotropy of each intermolecular potential energy
surface (PES) and the mechanism and energetics of the only available exothermic
reaction pathway, ${\rm CH}_{3}X+A\rightarrow{\rm CH}_{3}+AX$. The region of
the transition state is explored using two-dimensional PES cuts and estimates
of the activation energies are inferred. Nearly all combinations of methyl
halide and alkali-metal atom have positive barrier heights, indicating that
reactions at low temperatures will be slow.

Chemical Physics