Terms representing the reorganization of bonding within charge‐bond order matrices of reacting molecules

The usual way of obtaining charge‐bond order (CBO) matrices of molecules by summing up the MO LCAO coefficients over occupied molecular orbitals (MOs) is extended to derive terms representing the reorganization of bonding in reacting systems. The CBO matrix of a certain molecule (reactant) under influence of another one (reagent) is expressed in the form of power series with respect to intermolecular interaction. Terms of this series responsible for the internal reorganization of bonding in the reactant are also shown to be representable by sums of MO LCAO coefficients of the relevant isolated compound. As opposed to the case of a single molecule, the new sums embrace all MOs of the reactant and their pairs. This result is conditioned by the fact that the actual occupation numbers of MOs differ from either two or zero in the bimolecular system because of the intermolecular charge transfer, and bond orders arise between pairs of MOs in addition. Partial increments to the final reorganization of bonding related to individual MOs and to their pairs are then studied separately. These increments may be classified on the basis of criteria applied to MOs they originate from. In particular, symmetric and antisymmetric increments are distinguished with respect to any symmetry operation of the isolated reactant lost under influence of an approaching reagent. Increments of the same symmetry are subsequently collected into separate groups representable by specific graphical schemes. Consequently, the final pattern of charge and bond order redistribution in the reactant under influence of an approaching reagent follows from superposition of a few principal schemes. The results are illustrated by consideration of specific examples, in particular of addition of electrophile to the butadiene molecule. © 2009 Wiley Periodicals, Inc. Int J Quantum Chem, 2010