Steric hindrance can be probed via the dependence of the reactivity on reagent rotation: O+HCl

Kinematic considerations imply that the steric hindrance due to a "bulky" group can be directly probed using rotationally excited reactants. At typical collision and angular velocities, a shadow due to the bulky group restricts entry into the cone of acceptance for rotationally excited reactants. The conclusion is discussed for the realistic case where the anisotropy of the long range potential can reorient the incoming trajectories. Exact and approximate (reactant j_z conserving) classical trajectory computations for the O+HCl and O+DCl reactions, on two different potential energy surfaces are used as an illustration.