Jane S. Murray

Directional Noncovalent Interactions: Repulsion and Dispersion.

The interaction energies between an argon atom and the dihalogens Br2, BrCl, and BrF have been investigated using frozen core CCSD(T)(fc)/aug-cc-pVQZ calculations as reference values for other levels of theory. The potential-energy hypersurfaces show two types of minima: (1) collinear with the dihalogen bond and (2) in a bridging position. The former represent the most stable minima for these systems, and their binding energies decrease in the order Br > Cl > F. Isotropic atom-atom potentials cannot reproduce this binding pattern. Of the other levels of theory, CCSD(T)(fc)/aug-cc-pVTZ reproduces the reference data very well, as does MP2(fc)/aug-cc-pVDZ, which performs better than MP2 with the larger basis sets (aug-cc-pVQZ and aug-cc-pvTZ). B3LYP-D3 and M06-2X reproduce the binding patterns moderately well despite the former using an isotropic dispersion potential correction. B3LYP-D3(bj) performs even better. The success of the B3LYP-D3 methods is because polar flattening of the halogens allows the argon atom to approach more closely in the direction collinear with the bond, so that the sum of dispersion potential and repulsion is still negative at shorter distances than normally possible and the minimum is deeper at the van der Waals distance. Core polarization functions in the basis set and including the core orbitals in the CCSD(T)(full) calculations lead to a uniform decrease of approximately 20% in the magnitudes of the calculated interaction energies. The EXXRPA+@EXX (exact exchange random phase approximation) orbital-dependent density functional also gives interaction energies that correlate well with the highest level of theory but are approximately 10% low. The newly developed EXXRPA+@dRPA functional represents a systematic improvement on EXXRPA+@EXX.

An Overview of σ-Hole Bonding, an Important and Widely-Occurring Noncovalent Interaction

σ-Hole bonding is a highly directional noncovalent interaction between a positive region on a covalently-bonded Group IV–VII atom and a negative site on another molecule, e.g., a lone pair of a Lewis base. The positive region reflects the electron deficiency in the outer lobe of the p-type orbital involved in the covalent bond and is along the extension of this bond. There is now considerable experimental and computational evidence for σ-hole bonding. Within a given group, and for a particular Lewis base, the strength of the interaction increases with the polarizability of the atom and the electron-withdrawing power of the remainder of the molecule. For Groups IV–VI, there can be more than one σ-hole on the atom. For a series of Group IV–VII molecules, we give computed values of the positive surface electrostatic potentials associated with the σ-holes, and we also present some calculated interaction energies with different Lewis bases. σ-Hole bonding is competitive with hydrogen bonding, but the two can also accompany each other. The positive σ-holes usually exist in conjunction with negative regions on the remainders of the atoms’ surfaces, so that interactions with electrophiles as well as nucleophiles are possible. It is, therefore, not valid to assign single global atomic charges in these instances. Examples of σ-hole bonding in molecular biology and crystal engineering are discussed.

Non-hydrogen-Bonding Intramolecular Interactions: Important but Often Overlooked

Our focus in this chapter is upon intramolecular noncovalent interactions, electrostatically driven, but not including hydrogen bonding. They often involve a positive σ-hole or π-hole on a covalently-bonded Group IV – VII atom, in conjunction with a negative site in the molecule. Examples are given involving NO2 groups, Si–O–N bond angles, and specific 1,3-Si—O, 1,4-S—O, 1,4-Se—O, 1,3-P—Cl and 1,4-C—O interactions. These examples demonstrate that intramolecular interactions can play significant roles in determining the structure of a molecule and also its reactive properties. This often involves stabilizing a particular conformation, but can also include markedly affecting bond lengths and/or angles. It is essential to take intramolecular interactions into account in trying to understand and predict molecular behavior, and furthermore to exploit them in designing new materials, in pharmacology, crystal engineering, etc.