Fundamental Aspects of Recoupled Pair Bonds. III. The Frustrated Recoupled Pair Bond in Oxygen Monofluoride

New Insights into the Remarkable Difference between CH5- and SiH5. J Phys Chem A 2021;125:7414-7424. [PMID: 34424705 DOI: 10.1021/acs.jpca.1c05357]

It has long been known that there is a fundamental difference in the electronic structures of CH₅^- and SiH₅^-, two isoelectronic molecules. The former is a saddle point for the S_N2 exchange reaction H^- + CH₄ → [CH₅^-]^‡ → CH₄ + H^-, while the latter is a stable molecule that is bound relative to SiH₄ + H^-. SCGVB calculations indicate that this difference is the result of a dramatic difference in the nature of the axial electron pairs in these anions. In SiH₅^-, the axial pairs represent two stable bonds-a weak recoupled pair bond dyad. In CH₅^-, the axial electron pairs represent an intermediate transition between the electron pairs in the reactants and those in the products. Furthermore, the axial orbitals at the saddle point in CH₅^- are highly overlapping, giving rise to strong Pauli repulsion and a high barrier for the S_N2 exchange reaction.

Spin-Coupled Generalized Valence Bond Theory: New Perspectves on the Electronic Structure of Molecules and Chemical Bonds

Spin-Coupled Generalized Valence Bond (SCGVB) theory provides the foundation for a comprehensive theory of the electronic structure of molecules. SCGVB theory offers a compelling orbital description of the electronic structure of molecules as well as an efficient and effective zero-order wave function for calculations striving for quantitative predictions of molecular structures, energetics, and other properties. The orbitals in the SCGVB wave function are usually semilocalized, and for most molecules, they can be interpreted using concepts familiar to all chemists (hybrid orbitals, localized bond pairs, lone pairs, etc.). SCGVB theory also provides new perspectives on the nature of the bonds in molecules such as C₂, Be₂ and SF₄/SF₆. SCGVB theory contributes unparalleled insights into the underlying cause of the first-row anomaly in inorganic chemistry as well as the electronic structure of organic molecules and the electronic mechanisms of organic reactions. The SCGVB wave function accounts for nondynamical correlation effects and, thus, corrects the most serious deficiency in molecular orbital (RHF) wave functions. Dynamical correlation effects, which are critical for quantitative predictions, can be taken into account using the SCGVB wave function as the zero-order wave function for multireference configuration interaction or coupled cluster calculations.

Hypervalent Bonding in the OF(a4Σ-), SF(a4Σ-), SF5/SF6, and OSF4 Species

Hypervalency has struggled the conventional wisdom of too many chemists for so many years. Numerous theories and bonding models have been introduced but the so-called "hypervalency" mystery remains. We offer a simple and appealing explanation for the bonding mechanism of OF(a⁴Σ^-), SF(a⁴Σ^-), SF₅/SF₆, and OSF₄ based solely on the fact that excited and/or ionic states of the constituent fragments may and actually do occur in the ground states of so many "every day" molecules. In particular, and through multireference methods, we have found that the bonding in all the studied species is ionic in nature, perhaps contrary to the present status of our chemical beliefs. Although the "atoms in molecules" hypothesis is certainly not the only way to explain the formation of the chemical bond, we strongly believe that it is the simplest and most economical conceptual principle that should guide our chemical thinking.