Chemistry Reference
In-Depth Information
The equilibrium angular geometry of a halogen-bonded complex B
XY can
be predicted by assuming that the internuclear axis of a XY or X
2
molecule
lies:
1. Along the axis of a non-bonding (n) electron pair carried by the acceptor
atom Z of B, with order of atoms Z
···
···
δ
+
X - Y
δ
-
,or
2. Along the local symmetry axis of a
π
or pseudo-
π
orbital if B carries only
-pairs, or
3. Along the axis of a n-pair when B carries both n- and
π
π
-pairs (i.e. rule 1
takes precedence)
The main difference between hydrogen bond and the halogen bond lies in the
propensity of the hydrogen bond to be non-linear [28, 29], when symmetry
of the complex is appropriate (molecular point group C
S
or C
1
). In so far as
complexes B
Cl - F, where Z is the accep-
tor atom/centre in B, appear to be nearly collinear in all cases, while the nuclei
Z
···
ClF are concerned, the nuclei Z
···
HCl of appropriate symmetry often show signifi-
cant deviations from collinearity. This propensity for the hydrogen-bonded
species B
···
H - Cl in complexes B
···
···
HCl to exhibit non-linear hydrogen bonds can be understood as
follows.
We imagine that
δ
+
H-Cl
δ
-
approaches B,
δ
+
H first, along the axis of, e.g.,
an n-pair, as required by the rules. Then a secondary attraction, between the
nucleophilic end Cl
δ
-
of HCl and the most electrophilic region E of B, causes
Cl
δ
-
to move towards E but with
δ
+
H fixed, so that the motion is pivoted at
δ
+
H. The angle Z
in most of the figures) therefore re-
mains constant in first approximation, which explains why the values of
···
H - Cl (defined as
φ
φ
in
complexes B
HCl are those predicted by the rules even though the hydro-
gen bond is non-linear. In the new equilibrium position the force of attraction
between E and Cl
δ
-
is balanced by the force tending to restore the hydrogen
bond to linearity. There are three factors that conspire to keep the Z
···
···
Cl - F
nuclei in B
···
ClF more nearly collinear than the nuclei Z
···
H - Cl in the cor-
responding complex B
···
HCl:
1. For a given B, the Z
···
Cl bond in B
···
ClF is stronger than the Z
···
Hbond
HCl (as measured by
k
σ
) and is presumably more difficult to bend
2. Cl
δ
-
in HCl is probably a better nucleophile than F
δ
-
of ClF
3. F
δ
-
in B
···
is further away from the electrophilic region E of B than is Cl
δ
-
(see
Sect. 3.4)
It is of interest to note that systematic studies [200-204] of complexes
B
···
HCCH involving weak primary hydrogen bonds Z
···
HCCH have re-
φ
vealed large non-linearities, but with an angle
that remains reason-
ably close to those predicted by the rules. Figure 22 illustrates this result
through the experimentally determined geometries for the cases when B is
2,5-dihydrofuran [200], oxirane [201], formaldehyde [202], thiirane [203],
and vinyl fluoride [204]. On the other hand, as noted in Sect. 3.1.3, both
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