Selection Rules for Chemical Reactions Using the Orbital Phase Continuity Principle

Some recent ab initio calculations on simple molecular reactions are examined and found to indicate that in order for a reaction to have a low-activation energy certain phase relationships must occur between the orbitals of the reactants and the orbitals of the products. Herein this simple principle is developed and applied to several reactions of conjugated and nonconjugated molecules. For thermal reactions, the resulting selection rules are in general agreement with the generalized Woodward-Hoffmann rules [some exceptions occur for reactions involving open-shell molecules, e.g., O₂(1Δg) and NH(1Δ)] and with experiment. However, the orbital phase continuity principle (OPCP) does not depend upon molecular symmetry and hence can be applied easily to reactions involving no symmetry. The OPCP is based on the generalized valence bond (GVB) self-consistent field method, which leads to orbitals more akin to the valence bond description than to the Hartree-Fock or molecular orbital description. Thus, in a sense the use of OPCP for selection rules for chemical reactions might be considered the valence-bond analog of the Hoffmann-Woodward approach (for MO wave functions).