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 - Halogen Bonding: Fundamentals and Applications - page 17

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 for-
mation of B ··· XY) is first outlined. A comparison of the angular geometries of the pair
of halogen-bonded and hydrogen-bonded complexes B HCl as B is sys-

tematically varied follows. Systematic relationships among the radial geometries of the
two series are also summarised. The intermolecular stretching force constants k σ

··· ClF and B ···

and the

extent of electron transfer (both inter- and intramolecular) on formation of B XY, for

XY = Cl 2 ,Br 2 ,ClF,BrClorICl,areshowntovarysystematicallyasBisvaried.Astrik-
ing 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 ap-
plied 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 halo-
gen bond tends to remain close to linearity. A model for the halogen bond, successfully
applied earlier to the hydrogen bond, is proposed.

···

Keywords Lewis bases · Dihalogens · Halogen bond · Angular geometry ·

Electric charge transfer

Abbreviations
Efg Electric field gradient

n-pair Non-bonding electron pair

π -pair π -bonding electron pair

XY Generalised dihalogen molecules

HX Generalised hydrogen halide

1
Introduction

···

This chapter is restricted to a discussion of halogen-bonded complexes B XY

that involve a homo- or hetero-dihalogen molecule XY as the electron acceptor
and one of a series of simple Lewis bases B, which are chosen for their simplicity
and to provide a range of electron-donating abilities. Moreover, we shall restrict
attention to the gas phase so that the experimental properties determined refer
to the isolated complex. Comparisons with the results of electronic structure
calculations are then appropriate. All of the experimental properties of iso-
lated complexes B

XY considered here result from interpreting spectroscopic
constants obtained by analysis of rotational spectra.

···

1.1
Historical Background

The first report of an adduct of the type to be discussed here was that
of Guthrie in 1863 [1], who described the compound H 3 N I 2 .Thespec-

troscopy of the interaction of benzene with molecular iodine in the UV/visible

···