Chemistry Reference
In-Depth Information
Fig. 17
The n-pair/
-pair model of vinyl fluoride and scale drawings of the experimental
geometries of vinyl fluoride
π
···
···
ClF. Note that rule 3 is obeyed,
with the n-pair taking precedence over the
π
-pair in defining the angular geometry in
both cases. The angles C
1
-F
HClandvinylfluoride
Cl of the HCl and ClF complexes, respec-
tively, are similar, but the non-linearity of the hydrogen bond is large compared with
that of the halogen bond, which is negligible. See Fig. 1 for key to the colour coding
of atoms
···
HandC
1
-F
···
Furan is the prototype of molecules that carry both non-bonding and
aromatic
elec-
tron density in this molecule is shown in Fig. 18. The oxygen atom is taken
to have a non-bonding electron pair in an orbital whose symmetry axis coin-
cides with the C
2
axis of furan.
If rule 3 is applied to complexes of furan with HCl or ClF, the pre-
dicted geometry would be one that retains C
2
v
symmetry, with the HCl or
ClF molecule lying along the axis of the n-pair, and hence along the fu-
ran C
2
axis, with
δ
+
Hor
δ
+
Cl, respectively, nearest to O. The experimentally
determined geometry of furan
π
bonding electron pairs. The usual model for the n-pair and
π
HCl [165], which is shown in Fig. 18, is
indeed precisely as predicted, as is that of furan
···
···
HF [166]. The geom-
etry of furan
ClF [81], established by rotational spectroscopy, is strik-
ingly different, as may be seen from Fig. 18. It is obviously not the ana-
logue of that obtained for furan
···
δ
+
Cl of ClF ap-
···
HCl. Instead, the end
pears to interact with the
electron density associated with carbon atoms
C
2
and C
3
, so that this geometry violates rule 3. When furan was re-
placed by its sulfur analogue thiophene,
both
thiophene
π
···
HCl [167] and
···
thiophene
ClF [83] were shown to posses a face-on geometry and both vi-
olate rule 3. Thiophene
···
HF [168] and thiophene
···
HBr [169] have face-on
geometries similar to that of thiophene
···
HCl.
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