Halogen bonding (XB) is a type of noncovalent interaction between a halogen atom X in one molecule and a negative site in another. X can be chlorine, bromine or iodine. The strength of the interaction increases in the order Cl

In the past several years, halogen bonds have been shown to be relevant in crystal engineering and biomedical applications. One of the reasons for the utility of these types of noncovalent interactions in the development of, for example, pharmaceutical ligands is that their strengths and geometric properties are very tunable. That is, substitution of atoms or chemical groups in the vicinity of a halogen can have a very strong effect on the strength of the halogen bond. In this study we investigate halogen-bonding interactions involving aromatically-bound halogens (Cl, Br, and I) and a carbonyl oxygen. The properties of these halogen bonds are modulated by substitution of aromatic hydrogens with fluorines, which are very electronegative. It is found that these types of substitutions have dramatic effects on the strengths of the halogen bonds, leading to interactions that can be up to 100% stronger. Very good correlations are obtained between the interaction energies and the magnitudes of the positive electrostatic potentials (σ-holes) on the halogens. Interestingly, it is seen that the substitution of fluorines in systems containing smaller halogens results in electrostatic potentials resembling those of systems with larger halogens, with correspondingly stronger interaction energies. It is also shown that aromatic fluorine substitutions affect the optimal geometries of the halogen-bonded complexes, often as the result of secondary interactions.

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Covalently bonded atoms, at least in Groups V-VII, may have regions of both positive and negative electrostatic potentials on their surfaces. The positive regions tend to be along the extensions of the bonds to these atoms; the origin of this can be explained in terms of the sigma-hole concept. It is thus possible for such an atom in one molecule to interact electrostatically with its counterpart in a second, identical molecule, forming a highly directional noncovalent bond. Several examples are presented and discussed. Such "like-like" interactions could not be understood in terms of atomic charges assigned by any of the usual procedures, which view a bonded atom as being entirely positive or negative.

Halogen bonding is a unique type of noncovalent binding phenomenon in which a halogen atom interacts attractively with an electronegative atom such as oxygen or nitrogen. These types of interactions have been the subject of many recent investigations because of their potential in the development of new materials and pharmaceutical compounds. Recently, it was observed that most halogen bonding interactions in biological contexts involve close contacts between a halogen bound to an aromatic ring and a carbonyl oxygen on a protein's backbone structure. In this work we investigate interactions of substituted bromobenzenes and bromopyrimidines with acetone to ascertain the effects of various substituents upon the strengths of these interactions. It was found that replacement of ring hydrogens in these systems has dramatic effects upon the interaction strengths of the resulting complexes, which have interaction energies between -1.80 and -7.11 kcal/mol. Examination of the electrostatic potentials of the substituted bromobenzene and bromopyrimidine monomers indicates that the addition of substituents has a large influence upon the most positive electrostatic potential on the surface of the interacting bromine and thus modulates these halogen bonding interactions. Results obtained using the symmetry-adapted perturbation theory (SAPT) interaction energy decomposition procedure also indicate that electrostatic interactions play the key role in these halogen bonding interactions. These results have important implications in drug design and crystal engineering. Halogen bonds have been a subject of great interest in these fields because of their unique noncovalent bonding characteristics.

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In some halides RX, the halogen X has a region of positive electrostatic potential on its outermost portion, centered around the extension of the R-X bond. The electrostatic attraction between this positive region and a lone pair of a Lewis base is termed halogen bonding. The existence and magnitudes of such positive potentials on some covalently bonded halogens, and the characteristic directionality of the interaction, can be explained in terms of the degree of sp hybridization and polarizability of X and the electronegativity of R. Halogen bonding increases in strength in the order Cl < Br < I; fluorine is frequently said to not form halogen bonds, although a notable result of the present study is computational evidence that it does have the capability of doing so, if R is sufficiently electron withdrawing. An increasingly important application of halogen bonding is in the design of new materials (e.g., crystal engineering). In this paper, we present the calculated energies of a series of halogen-bonding interactions that could be the basis for forming linear chains, of types X----X----X---- or X----Y----X----Y----. We focus upon chlorides and bromides, and nitrogen bases. The B3PW91/6-311G(3df,2p) and MP2/6-311++G(3df,2p) procedures were used. We show how the computed electrostatic potentials (B3PW91/6-31G**) can provide guidance in selecting appropriate halide/base pairs.

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