Direct mapping of the angle-dependent barrier to reaction for Cl + CHD 3 using polarized scattering data.

The transition state, which gates and modulates reactive flux, serves as the central concept in our understanding of activated reactions. The barrier height of the transition state can be estimated from the activation energy taken from thermal kinetics data or from the energetic threshold in the measured excitation function (the dependence of reaction cross-sections on initial collision energies). However, another critical and equally important property, the angle-dependent barrier to reaction, has not yet been amenable to experimental determination until now. Here, using the benchmark reaction of Cl + CHD3 (v1  = 1) as an example, we show how to map this anisotropic property of the transition state as a function of collision energy from the preferred reactant bond alignment of the backward-scattered products-the imprints of small impact-parameter collisions. The deduced bend potential at the transition state agrees with ab initio calculations. We expect that the method should be applicable to many other direct reactions with a collinear barrier.