Halogen acceptors in hydrogen bonding

This review provides a comparative assessment of hydrogen bonding (HB) interactions with halogen (X) acceptors. Based on the data available in the literature we selected various inter- and intramolecular systems, for which (possibly) all the four halogen derivatives have been studied either by experimental or theoretical methods. Four main acceptor types have been considered in our assessment, viz., free X^- anions, halogens involved in MX (M = transition metal), HX and CX bonds. The hydrogen donors in the HB systems (DH) included the simple H₂O, NH₃, HX, H ₂ molecules, various alcohols, amines as well as aliphatic and aromatic CH groups. Among the molecular properties we focussed on the HB energy and structural characteristics (at the first place on the H⋯X distance), which are particularly informative on the HB propensities of the halogens. Where available, the shift of the DH stretching frequency (ν_DH) involved in the HB interaction is also discussed.