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Covalently-bonded atoms of Groups IV-VII tend to have anisotropic charge distributions, the electronic densities being less on the extensions of the bonds (σ-holes) than in the intervening regions. These σ-holes often give rise to positive electrostatic potentials through which the atom can interact attractively and highly directionally with negative sites (e.g., lone pairs, π electrons and anions), forming noncovalent complexes. For Group VII this is called-halogen bonding‖ and has been widely studied both computationally and experimentally. For Groups IV-VI, it is only since 2007 that positive σ-holes have been recognized as explaining many noncovalent interactions that have in some instances long been known experimentally. There is considerable experimental evidence for such interactions involving groups IV and VI, particularly in the form of surveys of crystal structures. However we have found less extensive evidence for Group V. Accordingly we have now conducted a survey of the Cambridge Structural Database for crystalline close contacts of trivalent nitrogen, phosphorus and arsenic with six different types of electronegative atoms in neighboring molecules. We have found numerous close contacts that fit the criteria for σ-hole interactions. Some of these are discussed in detail; in two instances, computed molecular electrostatic potentials are presented.

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Stacking interactions between pyridine molecules and the influence of simultaneous hydrogen bonds were studied by analyzing data in the Cambridge Structural Database (CSD) and by ab initio calculations. The results show remarkably stronger stacking interactions of pyridines with hydrogen bonds, because of local parallel alignment interactions of OH bonds with the aromatic ring. Data in the crystal structures from the CSD and ab initio calculations show that normal distances (R) in stacking interactions of pyridines with simultaneous hydrogen bonds are shorter than those in stacking interactions without simultaneous hydrogen bonds. Furthermore, the calculated binding energies for stacking are substantially stronger when the pyridines have hydrogen bonds; the binding energy of the stacking interaction between pyridine−water dimers is −6.86 kcal/mol, while that between pyridines is −4.08 kcal/mol. Surprisingly, in the minimum energy structure of the stacked pyridine−water dimers, the contribution of the local parallel-alignment interactions between water and the other pyridine (−2.98 kcal/mol) is slightly larger than the contribution of the stacking interaction between two pyridine molecules (−2.67 kcal/mol). The local influence of hydrogen bonds on stacking, via parallel alignment interactions, can be very important for all systems with heteroaromatic molecules and groups, especially DNA and RNA.

The geometry of hydrogen bonds in the crystal structures from the Cambridge Structural Database and calculated data show that water coordination to a metal ion has a remarkable influence on hydrogen bonds. The calculated energies of hydrogen bonds of coordinated water are much stronger, even if the aqua complex is neutral.