Chemistry Reference
In-Depth Information
u ¼
180
) and
is unable to discriminate between H-bonded (
u ¼
0
) configurations, giving in both cases
anti-H-bonded (
2
HF
ð
6
Þ¼
2
a
m
He
E
ind
2
ð
11
:
57
Þ
R
6
The next term in R
7
, which implies further polarization of the He
atom by the mixed dipole-quadrupole moments of HF, contains a
cos
3
term:
10
u
3
a
m
HF
Q
He
HF
E
ind
cos
3
ð
7
Þ¼
u
ð
11
:
58
Þ
2
R
7
u ¼
180
),
so that we can appropriately speak of formation of an H-bond
between He and HF (Magnasco et al., 1989).
which stabilizes theH-bonded configurationHe
H-F (
11.4 VAN DER WAALS AND HYDROGEN BONDS
From all we have seen so far, we can say that a VdW bond occurs when
the small Pauli repulsion arising from the first-order interaction of
closed-shell molecules at long range is overbalanced by weak attractive
second-order induction and dispersion forces. VdWmolecules are weakly
bound complexeswith large-amplitude vibrational structure (Buckingham,
1982). This is the case of the dimers of the rare gasesX
2
(X
¼
He, Ne, Ar, Kr,
Xe) or the weak complexes between centrosymmetrical molecules like
(H
2
)
2
or (N
2
)
2
. Complexes between proton-donor and proton-acceptor
molecules, like (HF)
2
or (H
2
O)
2
, involve formation of hydrogen bonds
(H-bonds), which are essentially electrostatic in nature, and lie on the
borderline between VdWmolecules and 'good' molecules, having sensibly
larger intermolecular energies.
Figure 11.8 shows the VdWpotential curve resulting for the interaction
of two ground-state He atoms at medium range. The upper curve is the
first-order energy E
1
mostly arising from repulsive (Pauli) exchange-
overlap; the bottom curve is the resultant of adding the attractive energy
E
disp
2
due to London dispersion. A weak potential minimum (about
33
10
6
E
h
) is observed at the rather large distance of R
e
¼
5
:
6a
0
.It
10
Q
is the permanent quadrupole moment of HF.
