Chemistry Reference
In-Depth Information
H
θ
F
H
F
Figure 11.6 Experimentally observed structure for (HF)
2
interaction terms, going as R
4
, favour instead an L configuration
of the dimer, giving as a whole the structure of the homodimer with
u
60
observed by experiment and schematically represented in
Figure 11.6.
A similar result is obtained by the qualitative MO description of
the H-bond in (HF)
2
depicted in the drawings of Figure 11.7, which
showelectron transfer fromF lone pairs (doubly occupiedMOs) to a
vacant (empty MO) orbital on H. The top row shows
s
charge
transfer from F to H, the bottom row
charge transfer from F to H,
whichmust be doubled on account of the twofold degeneracy of the
p
p
p
x
MO). As a result, the dimer
roughly assumes the noncollinear geometrical structure
9
depicted in
Figure 11.6.
(b) As a further example, the leading R
6
term describing the polar-
ization of the He atom by the HF molecule, namely
energy level (the figure shows the
2
HF
3cos
2
ð
6
Þ¼
a
m
uþ
1
2
He
E
ind
2
ð
11
:
56
Þ
R
6
.
H
F
H
F
H
.
F
H
F
Figure 11.7 MO description of the origin of the H-bond in (HF)
2
9
It must be remarked that molecular beam experimentalists refer to the geometrical shape of
Figure 11.6 as a linear dimer.
