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 23

Attention will be paid to the systematic relationship of the geometries
of B ··· XY to those of hydrogen-bonded complexes in the corresponding se-

ries B HX, especially for angular geometries, which are dealt with in detail

in Sects. 3.1, 3.2 and 3.3. Radial geometries are treated only in summary
(Sect. 3.4) here, but a detailed analysis is available in [19].
Many complexes B

···

XY,whereBisaLewisbaseandXYisF 2 , [30-37],
Cl 2 [22, 38-48], BrCl [49-58], ClF [34, 59-85], Br 2 [86-92] or ICl [93-102],
have been investigated by means of their rotational spectra. Those in the
group B

···

ClF cover the largest range of Lewis bases B, mainly because ClF
contains only a single quadrupolar nucleus and the rotational spectra are
relatively simple. Except for F 2 , all the other dihalogen molecules contain
two quadrupolar nuclei and hence the rotational transitions of the B

···

XY
complexes display complicated nuclear quadrupole hyperfine structure. For
this reason, the complexes B

···

ICl investigated
have been limited mainly to those of relatively high symmetry (molecular
point groups C ∞ v ,C 2 v and C 3 v ), which simplifies the spectral analysis. Nec-
essarily, these complexes yield more information about the electric charge
redistribution that accompanies complex formation (Sect. 5).
It will be shown in Sect. 5.1 that the extent of electron transfer to XY from
B and the extent of electron transfer within XY when B

··· Cl 2 ,B ··· BrCl, B ··· Br 2 and B ···

XY is formed are
both small in most complexes so far investigated in the gas phase. Mem-
bers of this group also have small intermolecular stretching force constants
k σ and are weakly bound (see Sect. 4). Such complexes are therefore of the
Mulliken outer type and the discussion of geometries here will be limited
to these. There are a few complexes B

···

XY that exhibit significant electric
charge rearrangement and are strongly bound. This group can be categorised
as approaching the Mulliken inner complex limit and will be discussed in
Sect. 5.2.
The discussions of Sects. 3.1, 3.2 and 3.3 are structured by reference to a set
of rules that were proposed some years ago [103, 104] for rationalising the an-
gular geometries of hydrogen-bonded complexes of the type B

···

··· HX, where X

is a halogen atom. These rules are as follows:

The equilibrium angular geometry of a hydrogen-bonded complex B ··· HX

can be predicted by assuming that the axis of the HX molecule lies:
1. Along the axis of a non-bonding (n) electron pair carried by the acceptor
atom of B, with δ + Hclosertothen-pairthanX δ - ,or
2. Along the local symmetry axis of a orbital (with δ + Hin- π -orpseudo- π

-density) when B carries no n-pairs, or
3. Along the axis of a n-pair, when B carries both n- and

teracting with the π

π -pairs (i.e. rule 1

takes precedence over rule 2 in this case)