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Fig. 22
Experimentally determined geometries, drawn to scale, for a series of weak,
hydrogen-bonded complexes B
···
HCCH, where B is 2,5-dihydrofuran, oxirane, formalde-
hyde, thiirane or vinyl fluoride. The values of [
φ
and
θ
] are [57.8(18)
◦
and 16.2(32)
◦
],
[90.4(12)
◦
and 29.8(4)
◦
], [92.0(15)
◦
and 39.5(10)
◦
], [96.0(5)
◦
and 42.9(23)
◦
]and
[122.6(4)
◦
and 36.5(2)
◦
], respectively. The non-linearities of the hydrogen bonds are
large because the primary Z
···
H hydrogen bonds are weak. The exception is 2,5-di-
hydrofuran
bond and
the most electrophilic region of B is larger because the angle
φ
is smaller than for other
B, thus making the secondary interaction correspondingly weaker. See Fig. 1 for key to
thecolourcodingofatoms
···
HCCH, in which the distance between the centre of the ethyne
π
SO
2
···
HCl [28, 126] have negligible non-linearity of the
halogen and hydrogen bonds, respectively, even though weakly bound. Exam-
ination of Fig. 10 reveals that the F
δ
-
and Cl
δ
-
are far away from the centre S
δ
+
in each case and that, therefore, the linear arrangements are to be expected.
The rules for predicting angular geometries of halogen-bonded complexes
ClF [70] and SO
2
···
B
XY have recently received support from a wide ranging analysis of X-ray
diffraction studies in the solid state by Laurence and co-workers [205]. This
study not only confirms the validity of the rules in connection with complexes
B
···
XY, where XY is Cl
2
,Br
2
,I
2
,IClandIBr,withmanyLewisbasesBbut
also reinforces the conclusion that halogen bonds Z
···
···
X - Yshowasmaller
propensity to be non-linear that do hydrogen bonds Z
···
H -- X.
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