Alkane molecules are held together in the crystal state by purportedly weak homonuclear R-H···H-R dihydrogen interactions. In an apparent contradiction, the high melting points and vaporization enthalpies of polyhedranes in condensed phases require quite strong intermolecular interactions. Two questions arise: 'How strong can a weak C-H···H-C bond be?' and 'How do the size and topology of the carbon skeleton affect these bonding interactions?' A systematic computational study of intermolecular interactions in dimers of n-alkanes and polyhedranes, such as tetrahedrane, cubane, octahedrane or dodecahedrane, showed that attractive C-H···H-C interactions are stronger than usually thought. We identified factors that account for the strength of these interactions, including the tertiary nature of the carbon atoms and their low pyramidality. An alkane with a bowl shape was designed in the search for stronger dihydrogen intermolecular bonding, and a dissociation energy as high as 12 kJ mol⁻¹ is predicted by our calculations.

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Spin−orbit coupling (SOC) calculations are performed along the reaction pathway of the oxidation process, FeO+ + H2 → Fe+ + H2O (eq 1). Selection rules are derived for SOC between different spin situations, and are applied to understand the computed SOC patterns along the oxidation pathway, and their relationship to the electronic structure of the various species. The process involves two spin inversion (SI) junctions between sextet and quartet states:  near the FeO+/H2 cluster at the entrance channel, and near the Fe+/H2O cluster at the exit channel. The sextet−quartet SOC is significant at the reactant extreme (for FeO+), but decreases at the FeO+/H2 cluster and continues to decrease until it becomes vanishingly small between the 6D−4F states of Fe+ at the product extreme. The results show that while the quartet surface provides a low-energy path, the SI junctions reduce the probability of the oxidation process significantly. In agreement with the deductions of Armentrout et al.,2c the poor bond activation capability of the 6D ground state of Fe+ in the reverse reaction is accounted for by the inefficient 6D−4F state mixing due to the expected poor SOC between the respective 4s13d6 and 3d7 configurations. On the other hand, the 4F excited state of Fe+ can activate H2O more efficiently since it can lead to the insertion intermediate 4(HFeOH+) in a spin-conserving manner. Other findings of Schwarz et al. ,2a and Armentrout et al. 2c,d are discussed in the light of the SOC patterns. The importance of the SOC at the exit channel is highlighted by comparing the product distribution of the reaction (eq 1) with analogous reactions of MO+ species:  when the ground state M+ has a 4s13d n -1 (Fe+, Mn+) electronic structure as opposed to those cases where the ground state electronic structure is 3d n (Co+, Ni+) and where no spin inversion is required. Predictions based on the understanding of the SOC patterns are made and compared with appropriate experimental data.

The wide-ranging applications of the halogen bond (X-bond), notably in self-assembling materials and medicinal chemistry, have placed this weak intermolecular interaction in a center of great deal of attention. There is a need to elucidate the physical nature of the halogen bond for better understanding of its similarity and differences vis-à-vis other weak intermolecular interactions, for example, hydrogen bond, as well as for developing improved force-fields to simulate nano- and biomaterials involving X-bonds. This understanding is the focus of the present study that combines the insights of a bottom-up approach based on ab initio valence bond (VB) theory and the block-localized wave function (BLW) theory that uses monomers to reconstruct the wave function of a complex. To this end and with an aim of unification, we studied the nature of X-bonds in 55 complexes using the combination of VB and BLW theories. Our conclusion is clear-cut; most of the X-bonds are held by charge transfer interactions (i.e., intermolecular hyperconjugation) as envisioned more than 60 years ago by Mulliken. This is consistent with the experimental and computational findings that X-bonds are more directional than H-bonds. Furthermore, the good linear correlation between charge transfer energies and total interaction energies partially accounts for the success of simple force fields in the simulation of large systems involving X-bonds.

In recent years, external electric fields (EEFs) have captured some spotlight as novel effectors of chemical change. EEFs directly impact the structure of molecular systems. For example, aligning an electric field along a specific bond‐axis leads to either shortening or elongation of the bond (and ultimately bond breaking). Furthermore, EEFs enable unprecedented control over chemical reactivity. Orienting an electric field along the so‐called “reaction‐axis,” the direction in which the electrons reorganize during the conversion from reactant to product, leads to catalysis or inhibition and can induce mechanistic crossover from concerted to stepwise reactions. Off‐reaction‐axis orientation enables control over the stereoselectivity of reactions and disables forbidden–orbital mixing. Two‐directional fields enable control over both reactivity and selectivity. In this advanced review, we offer an overview of this rapidly evolving research field with a focus on the valence bond modeling of EEF effects and the insight it offers. A wide variety of examples will be considered and a link to the experiment will be made throughout. We end by offering some perspectives in which we postulate that, as experimental techniques continue to mature, EEFs could potentially become a generally applicable “zapping” tool to facilitate elaborate chemical syntheses. This article is categorized under: Structure and Mechanism > Reaction Mechanisms and Catalysis