Chemical reactions are dominated by local processes like deformations of the charge distribution of the reactants or a change of the spin rearrangements in it. The importance of contraction and polarization of electron distributions has been shown by Ruedenberg et al; that unpaired electrons are necessary for the formation of covalent bonds is basic knowledge. But the nature of van Vleck's valence state is far from clear as is the process of promotion of reactants into this state. Proper multi-configurational wave functions (FORS, CASSCF) allow a qualitatively correct description of reactions, but local processes are hidden when delocalized MOs are used. Conventional valence bond (VB) theory makes local processes visible but due to non-orthogonal basis functions VB configuration state functions (CSF) are non-orthogonal themselves and have distance dependent chemical characteristcs, covalent CSF may becom ionic and vice versa. CSFs made with orthogonal basis functions or orbitals are always orthogonal to each other and retain their charcteristics during reactions. When orthogonal fragment MOS (FMO) are used, only the active electrons located at the reactive centers are described in a VB fashion with atom centered orbitals, all other electrons occupy delocalized FMOs. Such FMOs can be obtained by orthogonal transformation of MCSCF MOs (Procrustes transformation). OVB is thus used to analyze FORS wave functions.<br/>OVB CSFs describe always either neutral or ionic charge distributions, deformation of molecular charge distributions results in a change of the ration of neutral and ionic CSFs in the wave function. Similarly, a change from local low-spin to local high-spin states is indicated by a change of the respective CI coefficients. OVB analysis of FORS wave functions was used to describe local process during dissociation of multiple bonds and of the insertion of fragments into covalent bonds; the origin of reaction barriers was explained and the role of symmetry was discussed.<br/>Bond dissociation and fragment recombination can follow different reaction paths, OVB analysis shows that the corresponding wave functions have completely different chemical characteristics, which is typical for diabatic wave functions. When the system changes from one streambed to another, or from one diabatic state to another, one observes a change of the valence state of the reactive atoms.

How orthogonal valence bond theory reveals local processes in chemical reactions

ACS Fall 2021

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