Chemistry Reference
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molecule XY that accompany formation of B
XY lead directly to the changes
in the efgs at X and Y. In turn, the changes in the efgs at X and Y can be inter-
preted in terms of a simple model to give quantitative information about the
electric charge redistribution within XY that attends formation of B
···
···
XY. We
briefly discuss how the extents of intermolecular electron transfer
δ
i
(B
→
X)
and intramolecular molecular electron transfer
Y) can be extracted
from the observed nuclear quadrupole coupling constants of X and Y. Townes
and Dailey [187] developed a simple model for estimating efgs at nuclei, and
hence nuclear quadrupole coupling constants, in terms of the contributions
from the electrons in a molecule such as XY. First, they assume that filled
inner shells of electrons remained spherically symmetric when a molecule
XY is formed from the atoms X and Y and, second, they make a similar as-
sumption for valence-shell s electrons. Accordingly, filled inner shells and
valence s electrons contribute nothing to efgs, which therefore arise only
from p, d, ... valence shell electrons. Moreover, because the contribution of
δ
p
(X
→
a particular electron to the efg at a given nucleus varies as
r
-3
,where
r
is
the instantaneous distance between the nucleus and the electron, only elec-
trons centred on the nucleus in question contribute significantly to the efg
at that nucleus.
We assume that, on formation of B
δ
i
(i = intermolecular)
of an electronic charge is transferred from the electron donor atom of Z of the
Lewis base B to the
n
p
z
orbital of X and that similarly a fraction
···
XY, a fraction
p
(p = po-
larisation) of an electronic charge is transferred from
n
p
z
of X to
n
p
z
of Y,
where
z
is the XY internuclear axis and
n
and
n
are the valence-shell principal
quantum numbers of X and Y. Within the approximations of the Townes-
Dailey model [187], the nuclear quadrupole coupling constants at X and Y
in the hypothetical equilibrium state of B
δ
···
XY can be shown [178] to be
given by:
χ
zz
(X) =
χ
0
(X) - (
δ
i
-
δ
p
)
χ
A
(X)
(5)
and
χ
zz
(Y) =
χ
0
(Y) -
δ
p
χ
A
(Y) .
(6)
In Eqs. 5 and 6,
χ
A
(X) are the coupling constants associated with
thefreemoleculeXYandthefreeatomX,respectively,andsimilardefi-
nitions hold for
χ
0
(X) and
χ
A
(Y).Thefreemoleculevaluesareknownfor
Cl
2
[188], BrCl [189], Br
2
[190] and ICl [93], as are the free atom coupling
constants for Cl, Br and I [191]. The equilibrium coupling constants
χ
0
(Y) and
zz
(X)
χ
zz
(Y) are not observables. The observed (zero-point) coupling constant
χ
aa
(X) for B
and
χ
zz
(X) onto the
principal inertia axis
a
resulting from the angular oscillation
···
χ
XY is the projection of the equilibrium value
of the XY
subunit about its own centre of mass when within the complex B
β
XY. If
the motion of the B subunit does not change the efgs at X and Y (which is
likely to be a good approximation here)
···
χ
aa
(X) and
χ
aa
(Y) are given by the
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