Hydrogen Bonds and Halogen Bonds – A Comparative Study

Abstract

The halogen and hydrogen bonds are analyzed and compared. These interactions may be classified as σ-hole bonds steered by electrostatic forces since the arrangement of the units forming complexes or greater aggregates is determined by the distribution of electrostatic potential at the molecular surfaces. However, for both interactions the effects connected with the electron charge density shifts as a result of complexation are also pronounced. Numerous common properties for the A–H⋯B hydrogen bond, HB, and the A–X⋯B (X is the halogen atom) halogen bond, XB, may be exhibited; the directionality of those interactions, the increase of the positive charge of H and X atoms after the formation of the bond; the simultaneous decrease of the H or X-atom volume and the increase of the negative charge of A-atom. In general, both interactions are ruled by the same mechanisms; hyperconjugation and the rehybridization process. There are also distinct properties of hydrogen and halogen bonds that partly result from the differences in the volumes of H and X-atoms. The most important is that the halogen atoms may act at the same time as the Lewis acid and as a Lewis base but an H-atom possesses one of these properties – it may act as the Lewis acid centre in hydrogen bonds or the Lewis base in hydride bonds. The theoretical results on HBs and XBs in this chapter are supported by numerous experimental results – mostly examples of crystal structures.