Chemistry Reference
In-Depth Information
etry of trimethylamine is assumed to be unchanged when F
2
approaches it
along the C
3
axis to form [(CH
3
)
3
NF]
+
F
-
, the observed ground-state mo-
ments of inertia of the three isotopomers (CH
3
)
3
14
N
···
F
2
, (CH
3
)
3
15
N
···
···
F
2
F
2
can be fitted to give the distances
r
(N - F) = 1.29(4)
A
and
r
(F - F) = 2.32(4)A, a result consistent with significant covalent char-
acter of the N - F bond, with a substantially lengthened F - Fbond,and
therefore with an ion-pair type of structure. Subsequent ab initio calcula-
tions [197-199] showed that this approach overestimates the ionic character,
largely because the trimethylamine geometry is significantly perturbed on
formation of the complex. If this perturbed geometry of trimethylamine is
used in place of the unperturbed geometry and the observed experimental
moments of inertia are refitted, the revised bond lengths involving fluorine
are
r
(N - F) =
and (CD
3
)
3
14
N
···
1.9
A
, which are in good agreement
with the ab initio values [199]. Evidently the (CH
3
)
3
N
1.7
A
and
r
(F - F) =
∼
∼
F
2
complex has a sig-
nificant ion-pair character. We conclude therefore that even in the gas phase
there are complexes, such as (CH
3
)
3
N
···
···
ClF and (CH
3
)
3
N
···
F
2
,forwhichthe
description “inner complex” is partially appropriate.
6
Conclusions: A Model for the Halogen Bond in B···XY
We have established in Sect. 3 a strong case to support the conclusion that
acomplexB
XY involving a given Lewis base B and a dihalogen molecule
XY has an angular geometry that is isomorphous with that of the corres-
ponding member of the series of hydrogen-bonded complexes B
···
···
HX. This
was achieved mainly by a comparison of pairs of complexes B
···
HCl and
B
ClF for a given B, coupled with the systematic variation of the Lewis
base, although there is also similar, but less complete, evidence from com-
parisons of other series B
···
XY, where X is Cl, Br or I and Y
is Cl or Br. The observed parallelism among angular geometries of B
···
HX and B
···
···
HX
and B
XY suggests that the empirical rules [103, 104] for predicting an-
gular geometries of hydrogen-bonded complexes B
···
···
HX can be extended
to halogen-bonded complexes B
XY. The polarity of the heteronuclear di-
halogen molecules ClF, BrCl and ICl is such that the more electropositive
atom of each pair, i.e. Cl, Br and I, respectively, carries a small net pos-
itive charge
···
δ
+
while the other atom carries a corresponding net negative
-
. Although the homonuclear dihalogen molecules F
2
,Cl
2
and Br
2
have no electric dipole moment, each has a non-zero electric quadrupole mo-
ment that can be represented by the following electric charge distribution:
δ
+
X
δ
-
δ
δ
charge
-
X
δ
+
. Thus we can envisage the partial positive charge
δ
+
associated with
the atom X in XY or X
2
as interacting with a n- or a
-electron pair on the
Lewis base B when we restate the rules for halogen-bonded complexes B
π
···
XY
as follows:
Search WWH ::
Custom Search