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 53

bound electron (n-type or -type) from B, the greater is the extent of elec-

tron transfer from B to XY on formation of B

π

··· XY. It is also clear that for all

members of the B Cl 2 series the intermolecular charge transfer is negligible,

except possibly for B = NH 3 . For a given B, the order of the extent

···

δ i e of elec-

tron transfer is Cl 2 <Br 2 ∼ δ i have also been calculated

using ab initio methods by several authors [132, 192-195]. In summary, these
ab initio calculations lead to values of

BrCl < ICl. Values of

i of the same order of magnitude as
those obtained experimentally and show similar trends as B and XY are var-
ied. The conclusion from both experiments and ab initio calculations is that
the extent of electron transfer is generally < 0.06 e ,exceptwhenB=NH 3 and
PH 3 and XY =BrCl and ICl.
The values of

δ

p also behave systematically, as shown in Fig. 21, in which
δ p is plotted against k σ

δ

for the various series B ··· XY. It is evident that, for

agivenXY, δ p is an approximately linear function of k σ and hence of the

strength of the interaction. Moreover, for a given B the order of

δ p is ICl

> BrCl Br 2 > Cl 2 , which is the order of the polarisabilities of the leading

atoms X in B

∼

··· XY and therefore seems reasonable from the definition (see

earlier) of δ p .

5.2
Do Mulliken Inner Halogen-Bonded Complexes Exist in the Gas Phase?

A detailed analysis of the halogen and nitrogen nuclear quadrupole coupling
constants for the series of hydrogen-bonded complexes(CH 3 ) 3- n H n N HX,

where n = 0 and 3 and X = F, Cl, Br and I, has allowed conclusions about
how the extent of proton transfer changes with both n and X. The work has
been reviewed in detail elsewhere [196] and only a summary is given here.
It was concluded that progressive methylation of ammonia, which leads to
a monotonic increase in the gas-phase proton affinity of the base, coupled
with a decrease in the energy change accompanying the gas-phase process
HX = H + +X - along the series X = F, Cl, Br and I, allows the Mulliken inner
complex [(CH 3 ) 3- n H n NH] +

···

X - ··· to become more stable than the Mulliken

outer complex (CH 3 ) 3- n H n N HX when X = Br and I and n = 0. In fact, the

extent of proton transfer was crudely estimated to be

···

∼ 0%, ∼ 60%, ∼ 80%and

∼ HX,whenXisF,Cl,BrandI,respectively,

a result which indicates that the proton is gradually transferred as HX be-
comes progressively easier to dissociate in the case when the proton affinity
of the base is greatest. Is there any evidence for Mulliken inner complexes
[BX] +

100%fortheseries(CH 3 ) 3 N ···

Y - ?
Evidence for a significant contribution from the ionic form [BX] +

···

Y - in

···

···

a gas-phase complex B XY was first deduced from the spectroscopic con-

stants of H 3 N ClF, as obtained by analysis of its rotational spectrum [63].

In particular, the value k σ = 34.3 Nm -1 of the intermolecular stretching force
constant (obtained from the centrifugal distortion constant D J in the man-

···