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

Abstract

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 F2, Cl 2, Br2, 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 σ and the extent of electron transfer (both inter- and intramolecular) on formation of B⋯XY, for XY = Cl2, Br2, 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.