A primer on qualitative valence bond theory – a theory coming of age

Why Woodward and Hoffmann? And Why 1965?

Previous publications in this series on the history of the development of the Woodward-Hoffmann rules revealed why Woodward and Hoffmann were prime candidates to solve the pericyclic no-mechanism problem. This publication explains why it was the collaborative team of R. B. Woodward and Roald Hoffmann who did solve this mechanistic problem in a series of five communications in the Journal of the American Chemical Society in 1965. That is, the reasons why Woodward and Hoffmann were the perfect team, and why their individual capabilities, experiences, and qualities provided the perfect synergy are described. In part, this was the right time and the right place for them both, but the synergies were fundamental, intrinsic and idiosyncratic as a collaborative pair. Their orbital symmetry rules provided the mechanism of all concerted pericyclic reactions including electrocyclizations, cycloadditions, and sigmatropic rearrangements. Why it was 1965 and not earlier is also discussed.

The Many Chemists Who Could Have Proposed the Woodward-Hoffmann Rules (Including Roald Hoffmann) But Didn't: The Theoretical and Physical Chemists†

It is a reasonable question to ask, why, as of 1965 when the five Woodward-Hoffmann communication appeared, did no other physical chemist or chemical physicist or theoretical chemist discover the orbital symmetry rules for all pericyclic reactions? Two theoretical chemists - Luitzen Oosterhoff (in 1961) and Kenichi Fukui (in 1964) had discovered portions of the orbital symmetry rules; their stories appear in the papers immediately preceding this paper which is Paper 5 in a 27-paper series on the history of Woodward-Hoffmann rules. Concise yet telling stories of 19 other chemists who could have, might have, perhaps even should have discovered the Woodward-Hoffmann rules are presented with explanations as to why they did not do so. Social, political, and scientific explanations will summarize the analyses.

Understanding the Nature of the CH···HC Interactions in Alkanes

To understand the dispersion stabilization of hydrocarbons in solids and of encumbered molecules, wherein CH···HC interactions act as sticky fingers, we developed here a valence bond (VB) model and applied it to analyze the H···H interactions in dimers of H2 and alkanes. The VB analysis revealed two distinct mechanisms of "dispersion." In the dimers of small molecules like H-H···H-H and H3CH···HCH3, the stabilization arises primarily due to the increased importance of the VB structures which possess charge alternation, e.g., C(+)H(-)···H(+)C(-) and C(-)H(+)···H(-)C(+), and hence bring about electrostatic stabilization that holds the dimer. This is consistent with the classical mechanism of oscillating dipoles as the source of dispersion interactions. However, in larger alkanes, this mechanism is insufficient to glue the two molecules together. Here, the "dispersion" interaction comes about through perturbational mixing of VB structures, which reorganize the bonding electrons of the two interacting CH bonds via recoupling of these electrons to H···H, C···C, and C···H "bonds." Finally, an attempt is made to create a bridge from VB to molecular orbital (MO) and local pair natural-orbital coupled electron pair approximation (LPNO-CEPA/1) analyses of the interactions, which bring about CH···HC binding.

A three-state effective Hamiltonian for symmetric cationic diarylmethanes

We analyze the low-energy electronic structure of a series of symmetric cationic diarylmethanes, which are bridge-substituted derivatives of Michler's Hydrol Blue. We use a four-electron, three-orbital complete active space self-consistent field and multi-state multi-reference perturbation theory model to calculate a three-state diabatic effective Hamiltonian for each dye in the series. We exploit an isolobal analogy between the active spaces of the self-consistent field solutions for each dye to represent the electronic structure in a set of analogous diabatic states. The diabatic states can be identified with the bonding structures in classical resonance-theoretic models of cyanine dyes. We identify diabatic states with opposing charge and bond-order localization, analogous to the classical resonance structures, and a third state with charge on the bridge. While the left- and right-charged structures are similar for all dyes, the structure of the bridge-charged diabatic state, and the Hamiltonian matrix elements connected to it, change significantly across the series. The change is correlated with an inversion of the sign of the charge carrier on the bridge, which changes from an electron pair to a hole as the series is traversed.

Hydrogen-abstraction reactivity patterns from A to Y: the valence bond way

"Give us insight, not numbers" was Coulson's admonition to theoretical chemists. This Review shows that the valence bond (VB)-model provides insights and some good numbers for one of the fundamental reactions in nature, the hydrogen-atom transfer (HAT). The VB model is applied to over 50 reactions from the simplest H + H(2) process, to P450 hydroxylations and H-transfers among closed-shell molecules; for each system the barriers are estimated from raw data. The model creates a bridge to the Marcus equation and shows that H-atom abstraction by a closed-shell molecule requires a higher barrier owing to the additional promotion energy needed to prepare the abstractor for H-abstraction. Under certain conditions, a closed-shell abstractor can bypass this penalty through a proton-coupled electron transfer (PCET) mechanism. The VB model links the HAT and PCET mechanisms conceptually and shows the consequences that this linking has for H-abstraction reactivity.