The chemical shape of molecules: An introduction to dynamical stereochemistry

The steric requirements of activated atom exchange reactions are examined in light of the detailed information becoming available from experiments and simulations. The results are interpreted in terms of the physical shape of the molecule, which governs the ability of the reagents to reach the barrier, and the chemical shape, i.e., the barrier location as a function of distance and orientation. These two shapes can be probed by varying the initial conditions and/or via isotope effects. The discussion begins with the simplest model, where both shapes are equivalent and spherical, which suffices to discuss the main feature, namely, the orientation dependence of the reactivity. Refinements are then introduced to account for orientation/disorientation of the reagents en route to the barrier, the enhancement/decline of the cross section upon rotational and/or vibrational excitation and branching ratios.