Directional Tendencies of Halogen and Hydrogen Bonds

Halogen bonding, R-X center dot center dot center dot B, and hydrogen bonding, R-H center dot center dot center dot B, are electrostatically-driven noncovalent interactions of covalently-bonded halogens R-X and hydrogens R-H with negative sites B. A significant difference between halogen and hydrogen bonding is that the former is typically near-linear (the R-X-B angles are close to 180 degrees), whereas the latter is more likely to be nonlinear (R-H-B angles sometimes considerably less than 180 degrees). In this work, we have looked at the properties of several R-Br center dot center dot center dot B and R-H center dot center dot center dot B complexes as functions of these angles. The differences in the directional tendencies in these interactions can be attributed to the presence of nonbonding valence electrons on the bromines, and their absence on hydrogen. We also found that for a given negative site, the halogen and hydrogen bonding interaction energies correlate very well with the positive electrostatic potentials created at it by R-X and R-H. This attests to the electrostatically-driven nature of these interactions. Overall, this study provides support for regarding both halogen and hydrogen bonding as subsets of sigma-hole interactions. (C) 2010 Wiley Periodicals, Inc. Int J Quantum Chem 110: 2823-2832, 2010