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 22

The final spectroscopic constants of particular interest here are the halo-
gen nuclear quadrupole coupling constants χ αβ (X) and χ αβ (Y) [27], where

α are to be permuted over the principal inertial axis directions a , b

and c . Halogen nuclei (with the exception of 19 F) have an intrinsic (or spin)
angular momentum, I X , with a spin quantum number I X ≥

and β

2 and there-
fore with a non-zero electric quadrupole moment Q X . The nuclear spin vector
I X can couple in only a limited number of discrete orientations to the rota-
tional angular momentum vector J of the molecular framework. Each allowed
orientation of the angular momentum and spin vector corresponds to dif-
ferent orientations of the nuclear electric quadrupole moment with respect
to the electric field gradient

3 /

E X at X and therefore to a different interac-
tion energy. Hence, rotational energy levels (and therefore transitions) carry
a hyperfine structure. Analysis of this nuclear quadrupole hyperfine struc-
ture gives various components (depending on the molecular symmetry) of
the nuclear quadrupole coupling tensor

∇

aretobeper-
muted over the principal inertial axes a , b and c . The importance of this
tensor follows from its definition

χ αβ (X), where α and β

2 V X

where Q X is
the conventional electric quadrupole moment of nucleus X and -

χ αβ (X) = - ( eQ X h ) ∂ /∂α∂β

2 V

/∂α∂β
is a component of the electric field gradient (efg) tensor at the nucleus X.
This direct proportionality of

∂

χ αβ (Y) to the efgs at the nu-
clei X and Y, respectively, means that the changes

χ αβ (X) and

∆ χ αβ (X) and ∆ χ αβ (Y) in

χ αβ (X) and χ αβ (Y) that accompany formation of B ··· XY lead directly to the

changes in the efgs at X and Y. Hence ∆ χ αβ (Y) carry quan-

titative information about the electric charge redistribution associated with
the process. We shall see in Sect. 5 that intra- and intermolecular electron
transfer on formation of B

∆ χ αβ (X) and

XY can be estimated from these quantities. For
an asymmetric rotor of C S symmetry, only one off-diagonal element ( ab or
ac normally) of the tensor

···

χ αβ (Y)isnon-zerobutitsvaluepro-
vides important information about the orientation of the XY subunit with
respect to the principal inertial axis system ( a , b , c ) in complexes B

χ αβ (X) or

··· XY (and

indeed of the HX subunit in hydrogen-bonded complexes B ··· HX [28, 29]).

This leads to an estimate of the deviation of the Z H -- X)

atoms from collinearity, where Z is the electron-donor atom of B, as discussed
in Sect. 3.

··· X - Y(orZ ···

3
Comparison of the Angular and Radial Geometries
of Halogen-Bonded Complexes B···XY
and their Hydrogen-Bonded Analogues B···HX

In this section, we discuss the observed geometries, both angular (the relative
orientation of the components B and XY in space) and radial (the distance
between B and XY at the observed orientation) of complexes B ··· XY.