Halides and Halogen Atoms as Hydrogen-Bond Acceptors

Halides and Halogen Atoms as Hydrogen-Bond Acceptors 

The halide ions are strong hydrogen-bond acceptors. The infrared spectroscopy data for halides show clearly the characteristic shifts associated with hydrogen-bond acceptors, in the sequence 

There appears to be no obvious reason why the order of the spectral shifts for Cl and F are reversed relative to the hydrogen-bond strengths, which are F C1, as expected from their electronegativities. The X-H- -A hydrogen-bond lengths are consistently shorter than for any other atom in the same row in the Periodic Thble, i.e.  

In fact, X-H - - Br is the only well-established hydrogen bond known to form with third-row elements [473]. 

Chloride ions are three- and four-coordinated. The neutron diffraction analyses of the amino acid hydrochlorides provided the data in Thble 11.1, and the more extensive data obtained from the X-ray analyses of hydrochlorides of nucleic acid constituents are given in Thble 11.2. The data indicate clearly that the chloride ion may be three- or four-coordinated. When three-coordinated, the bonds may be in planar or in pyramidal configuration, with no bimodal distribution. The ligands of four-coordinated chloride ions are only very approximately in tetrahedral con- 

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