Correlation Effects on the Relative Stabilities of Alkanes

The “alkane branching effect” denotes the fact that simple alkanes with more highly branched carbon skeletons, for example, isobutane and neopentane, are more stable than their normal isomers, for example, <i>n</i>-butane and <i>n</i>-pentane. Although <i>n</i>-alkanes have no branches, the “kinks” (or “protobranches”) in their chains (defined as the composite of 1,3-alkyl–alkyl interactionsincluding methine, methylene, and methyl groups as alkyl entitiespresent in most linear, cyclic, and branched alkanes, but not methane or ethane) also are associated with lower energies. Branching and protobranching stabilization energies are evaluated by isodesmic comparisons of protobranched alkanes with ethane. Accurate ab initio characterization of branching and protobranching stability requires post-self-consistent field (SCF) treatments, which account for medium range (∼1.5–3.0 Å) electron correlation. Localized molecular orbital second-order Møller–Plesset (LMO-MP2) partitioning of the correlation energies of simple alkanes into localized contributions indicates that correlation effects between electrons in 1,3-alkyl groups are largely responsible for the enhanced correlation energies and general stabilities of branched and protobranched alkanes

A Hückel Theory Perspective on Möbius Aromaticity

Heilbronner’s Hückel molecular orbital treatment of Möbius 4n−π annulenes is revisited. When uneven twisting in π-systems of small Möbius rings is accounted for, their resonance energies become comparable to iso-π-electronic linear alkenes with the same number of carbon atoms. Larger Möbius rings distribute π-twisting more evenly but exhibit only modest aromatic stabilization. Dissected nucleus independent chemical shifts (NICS), based on the LMO (localized molecular orbital)–NICS(0)_π index confirm the magnetic aromaticity of the Möbius annulenes considered

Why Do Two π‑Electron Four-Membered Hückel Rings Pucker?

Notwithstanding their two (i.e., 4<i>n</i> + 2) π electrons, four-membered ring systems, <b>1</b>–<b>4</b>, favor puckered geometries (<b>1a</b>–<b>4a</b>) despite the reduction in vicinal π overlap and in the ring atom bond angles. This nonplanar preference is due to σ → π* hyperconjugative interactions across the ring (A) rather than to partial 1,3-bonding (B). Electronegative substituents (e.g., F in C₄F₄²⁺) reduce the σ → π* electron delocalization, and planar geometries result. In contrast, electropositive groups (e.g., SiH₃ in C₄(SiH₃)₄²⁺) enhance hyperconjugation and increase the ring inversion barriers substantially

Potential-Dependent Generation of O₂^– and LiO₂ and Their Critical Roles in O₂ Reduction to Li₂O₂ in Aprotic Li–O₂ Batteries

Discharging of the aprotic Li–O₂ battery relies on the oxygen reduction reaction (ORR) producing Li₂O₂ in the positive electrode, which remains incompletely understood. Here, we report a mechanistic study of the Li-ORR on a model system, i.e., an Au electrode in a Li⁺ dimethyl sulfoxide (DMSO) electrolyte. By spectroscopic identification of the reaction intermediates coupled with density functional theory calculations, we conclude that the formation of O₂^– and LiO₂ in the Li-ORR critically depends on electrode potentials and determines the Li₂O₂ formation mechanism. At low overpotentials (> 2.0 V vs Li/Li⁺) O₂^– is identified to be the first surface intermediate, which diffuses into the bulk electrolyte and forms Li₂O₂ therein via a solution-mediated disproportionation mechanism. At high overpotentials (ca. 2.0–1.6 V vs Li/Li⁺) LiO₂ has been observed, which can rapidly transform to Li₂O₂ by further electro-reduction, suggesting a surface-mediated mechanism. The solution-mediated Li₂O₂ formation that can account for the widely observed toroid-shaped discharged Li₂O₂ particles has also been thoroughly examined. Thus, O₂^– formation controls the overall reaction onset potential, and LiO₂ formation demarcates the change from a solution- to surface-mediated reaction mechanism. The new findings and improved understandings of the Li-ORR in DMSO will contribute to the further development of aprotic Li–O₂ batteries