Chemistry Reference
In-Depth Information
ner outlined in Sect. 2 is large compared with that (ca. 25 Nm
-1
)expected
from the plot of
k
σ
versus
N
B
showninFig.19.Similarly,theCl-nuclear
quadrupole coupling constant is smaller in magnitude than those of more
weakly bound B
ClF complexes. A detailed analysis suggested [63, 68]
acontributionofH
3
NCl
+
···
F
-
···
of roughly 20%tothevalencebonddescrip-
tion of H
3
N
ClF.
In view of the fact that complete methylation of H
3
N
···
···
HX to give
(CH
3
)
3
N
HX leads to an increased extent of proton transfer from HX to the
base when X is Cl and essentially complete transfer when X is I, it seemed rea-
sonable to seek a more significant contribution from the ionic valence bond
structure [(CH
3
)
3
NCl]
+
···
F
-
in (CH
3
)
3
N
ClF by examining properties sim-
ilarly derived from its rotational spectrum [68].
It was found that (CH
3
)
3
N
···
···
ClF has a centrifugal distortion constant
D
J
consistent with the large value
k
σ
∼
···
70 Nm
-1
for the intermolecular stretch-
ing force constant. The distance
r
(N
Cl) = 2.090A,asobtainedbyiso-
topic substitution at N and Cl, is very short compared with that predicted
for an intermolecular N
···
Cl bond in an analogous complex in which lit-
tle ionic character is expected, for example HCN
···
···
ClF [64], which is weakly
Cl) = 2.639(3)
A
.TheClnuclear
quadrupole coupling constant of (CH
3
)
3
N
= 12.3 Nm
-1
)andhas
r
(N
bound (
k
σ
···
ClF is significantly smaller in
magnitude than expected of a weakly bound complex. A detailed analysis
of the observed coupling constant leads to an estimated contribution of ca.
60% for the ionic valence bond structure [(CH
3
)
3
NCl]
+
···
F
-
. In addition,
···
14
N nuclear quadrupole coupling constant of (CH
3
)
3
N
the
ClF is consis-
tent with a substantial (roughly 70%) contribution of the ion-pair form. It
should be emphasised that the models used to interpret the Cl and N nu-
clear quadrupole coupling constants were crude and that the percentage ionic
characters deduced thereby are only semi-quantitative. Nevertheless, there
is evidence of a substantial (
···
∼
50%) contribution from the ionic structure
[(CH
3
)
3
NCl]
+
F
-
ClF
appears to be intermediate between a Mulliken outer and inner complex.
These experimental conclusions are consistent with the results of ab initio
calculations [197, 198].
A detailed examination of the rotational spectrum (CH
3
)
3
N
···
in a valence-bond description. Hence, (CH
3
)
3
N
···
F
2
led [37]
to molecular properties that suggest that this complex too has significant ion-
pair character. Thus, the behaviour of the spectral intensity as a function
of microwave radiation power led to an estimate of
···
10 Dfortheelectric
dipole moment, a value which is an order of magnitude large than that (
∼
1 D)
expected on the basis of the vector sum of the component dipole moments
(i.e. with no charge transfer). The centrifugal distortion constant
D
J
is con-
sistent with a large intermolecular stretching force constant
k
σ
.Thevalue
of the
14
N-nuclear quadrupole coupling constant implies a substantial con-
tribution from [(CH
3
)
3
NF]
+
∼
F
-
, as do all the other properties mentioned.
Ifthecomplexisassumedtobeentirely[(CH
3
)
3
NF]
+
···
F
-
···
and the geom-
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