Molecular Orbital Analysis of the Orientation-Dependent Barrier to Direct Exchange Reactions

After a general introduction to the current state of potential energy functions for delineating the steric requirements of exchange reactions, we proceed to a qualitative, Walsh-diagram-based orbital analysis of the preferred geometry approach to the barrier to such reactions. Extended Hiickel calculations, perturbation theory, and a frontier orbital perspective are used to analyze the X + H2 (collinear, due to the controlling role of the singly occupied highest molecular orbital), X + HX, X + HY (also collinear), H + X2 (bent, due to interaction with a relatively low lying unfilled orbital), H + XY (a preference for direction of attack depending on relative electronegativities), M + X2, M + XY, M + HX (collinear approach slightly preferred), X + XY, and triplet O atom reactions.