The interaction of dihalogens and hydrogen halides with Lewis bases in the gas phase: An experimental comparison of the halogen bond and the hydrogen bond

This chapter is concerned exclusively with the experimentally determined properties of halogen-bonded complexes of the type B...XY in isolation in the gas phase and their relationship with those of the corresponding hydrogen-bonded complexes B...HX. B is one of a series of simple Lewis bases and XY is a homo- or hetero-dihalogen molecule F-2, Cl-2, Br-2, ClF, BrCl or ICl The method used to determine these properties (angular and radial geometry, binding strength, and the extent of electric charge redistribution on formation of B...XY) is first outlined. A comparison of the angular geometries of the pair of halogen-bonded and hydrogen-bonded complexes B...ClF and B...HCl as B is systematically varied follows. Systematic relationships among the radial geometries of the two series are also summarised. The intermolecular stretching force constants k(sigma) and the extent of electron transfer (both inter- and intramolecular) on formation of B...XY, for XY = Cl-2, Br-2, ClF, BrCl or ICl, are shown to vary systematically as B is varied. A striking similarity noted among the properties of halogen-bonded complexes B...XY and their hydrogen-bonded analogues B...HX demonstrates that rules for predicting the angular geometries of hydrogen-bonded complexes (and other generalisations) may also be applied to the halogen-bonded series, but with the caveat that while the hydrogen bond shows a propensity to be non-linear when B...HX has appropriate symmetry, the halogen bond tends to remain close to linearity. A model for the halogen bond, successfully applied earlier to the hydrogen bond, is proposed.