Revealing the stereospecific chemistry of the reaction of Cl with aligned CHD₃(ν₁ = 1).

The concept of geometrical constraints and steric hindrance in reactions is implanted deeply in a chemist's 'chemical intuition'. However, until now a true three-dimensional view of these steric effects has not been realized experimentally for any chemical reaction in full. Here we report the complete three-dimensional characterization of the sterics of a benchmark polyatomic reaction by measuring the dependence of the product state-resolved angular distributions on the spatial alignment of the reactive bond in a crossed molecular beam experiment. The results prove the existence of two distinct microscopic reaction mechanisms. Detailed analysis reveals that the origin of the stereodynamics in the HCl(ν = 0) + CD(3)(0(0)) product channel can be captured by a textbook line-of-centres collision model. In contrast, a time-delay pathway, which includes a sharp switch from in-plane to out-of-plane scattering in the forwards direction, appears to be operative in forming the excited HCl(ν = 1) + CD(3)(0(0)) product pair.