Hydrogen-bond distances to halide ions in organic and organometallic crystal structures: Up-to-date database study

Geometrical data on hydrogen bonds to halide ions are compiled from the currently available crystal structures. Hydrogen bonds from 25 donor types to fluoride, chloride, bromide and iodide ions are considered. Compared with earlier compilations, the increased data volume allows a finer subdivision of O-H and N-H donors, and the donors C-H, S-H and P-H can be included. For a given donor type, the hydrygen-bond distance typically increases by over 0.5 Angstrom from fluoride to chloride, 0.15 Angstrom from chloride to bromide and 0.25 Angstrom from bromide to iodide accepters. The strongest of the C-H donors considered, chloroform, forms hydrogen bonds with chloride ions with an average H ... Cl separation of only 2.39 Angstrom and an average C ... Cl separation of 3.42 Angstrom. The lengthening of the N-H covalent bond in hydrogen bonds to chloride ions is quantified from neutron diffraction data.