Valence-bond approach to conservation of symmetry in concerted reactions

Abstract

Generalized electron-pair wave functions are used to describe the electronic states involved in a concerted reaction process. For symmetry preserving reaction paths, it is shown that primitive symmetry labels for the electron pairs can be used to determine the nature of the interaction energy between reactant and product functions and to make an assessment of the relative heights of the potential energy barriers that would be encountered along various stereospecific reaction paths. A conservation of symmetry principle is introduced for the simplest form of electron-pair function, that based on the canonical valence-bond description. This provides a generalization of the Woodward-Hoffmann rules for the conservation of molecular orbital symmetry. Illustrative examples are considered and the limitations of the symmetry conservation principle are discussed.

Valence bond theory for chemical dynamics

“…Valence bond theory has been used as a framework for modeling chemical reaction dynamics since the early days of quantum mechanics 2, 3, 20–24, 28, 29, 40, 41, 45, 48, 50, 51, 67, 72, 79, 81, 86, 87, 89, 90, 92, 95–99, 101–104, 106, 112, 210–286. In part this may be explained by the fact that valence bond theory often provides a concise description of multireference character, and transition states are more likely than stable molecules to exhibit multireference character.…”

“…The method has proved to be a very efficient way to fit potential energy surfaces,312, 313 and it can be made even more efficient by using different amounts of data to interpolate different elements of the Hessian of the resonance integral314 and by adjusting the diagonal valence‐bond matrix elements as well as the off‐diagonal ones 320. Thus, the lessons learned from valence bond theory1–3, 13, 20–112, 210–286 can be imported into MCMM to provide improved representations of reactive potential energy surfaces.…”

Visual valence bond rules for chemical reactions

“…Here the CeH bond involved in the reaction is correlated with p bonds by superconjugation interactions, and the phase for CeH should dier from the nearest p bond according to the phase alternation postulate. We see that [1,5] migration is allowed and that the H would be expected to stay on the same side of the plane (suprafacial) during the reaction. Similarly, the [1,3] and [1,7] migrations are pictured in Figs.…”

“…Symmetry plays no direct role in the OPCP. Based on generalized electron-pair wave functions and the primitive symmetry classi®cation of the prinicpal structure of a molecular system, the VB approach to conservation of symmetry has been suggested [5]. The idea of permutational symmetry control was introduced to pericyclic reactions by van der Lugt and Oosterho [6,7] and by Mulder and coworkers [8±10], and photoinduced electrocyclic reactions were discussed in terms of the VB orbitals of a cyclic system.…”

Chemical reaction with two different elementary transition states

“…The VB method, including qualitative resonance theory, declares the chemical reaction as a resonance between reactant and product. Different VB interpretations were proposed during the last 70 years . Shaik et al proposed avoided crossing for location of TS and presentation of electronic wave function (EWF) of TS by equivalent contributions of the reactant and product Heitler–London forms …”