Kinematic mass effect in the dynamical stereochemistry of activated bimolecular reactions

The dynamics of elementary A + BC exchange reactions with restricted cones of acceptance can be very much dependent on the location of the BC center of mass. Other things being equal, the heavier B is, the wider is its cone of acceptance for reaction with A, for a nonrotating BC molecule. The reactivity will, however, significantly decline the higher the rotational energy of BC is as compared to the A-BC relative kinetic energy. The opposite conclusions apply for reaction with the C end. The conclusions are motivated by analytical considerations of the variation of the effective potential energy function upon isotopic substitution and by explicit potential maps for the O + HCl, Cl + HCl, F + H₂, Cl + H₂, and O + H₂ reactions. These demonstrate that the heavier B is, the more the potential gradient is directed so as to orient reagents into the cone of acceptance.