Chemistry Reference
In-Depth Information
Table 5.1
Best values of HOMO/LUMO orbital energies
from accurate
augccpVTZ-MO calculations
a
and estimated intermolecular interaction integral
b
(E
h
)
1
=
2
D
e
Da
2
Molecule
R
=
a
0
a
HOMO
a
LUMO
Da
jbj¼
10
3
H
2
6.50
0
:
59
ð
1
s
g
Þ
0
:
049
ð
1
s
u
Þ
0.639
6.19
10
3
N
2
8.00
0
:
61
ð
1
p
u
Þ
0
:
081
ð
4
s
g
Þ
0.691
11.61
10
3
HF
5.09
0
:
65
ð
1
p
Þ
0
:
030
ð
4
sÞ
0.680
62.26
10
3
H
2
O
5.40
0
:
51
ð
1b
1
Þ
0
:
029
ð
4a
1
Þ
0.539
52.69
a
Pelloni (2008) private communication to V.Magnasco.
the first-row hydrides XH
n
; and (2) the second-order interactions for the
dimers between a rare gas atom and HX, which will be examined later in
Section 5.4.
In the first case, the hydrogen bond and the shape resulting therefrom
for these dimers can be understood in terms of the long-range interactions
between the first few permanent multipole moments of the interacting
molecules (Magnasco et al., 1989b).
Equilibrium bond distances and electric properties (permanent mo-
ments
4
up to l
3 and isotropic dipole polarizabilities) of a few polar
molecules are collected in Table 5.2 (Magnasco et al., 2006). Data for
H
2
O are taken from recent accurate work by Torheyden and
Jansen (2006).
A few comments on Table 5.2 seem appropriate at this point. CO has
such a small dipole moment C
O
þ
that can be considered a 'quasi-
quadrupolar' molecule. NH
3
has large axial quadrupole and octupole
moments directed along the z symmetry axis. Besides by its dipole
moment, H
2
O is characterized by a rather small
¼
06ea
0
2
axial quad-
rupole moment and by a large transverse quadrupole moment
ð
0
:
Þ
Þ
perpendicular to the z symmetry axis, mostly due to the couple of lone pair
electrons. LiH has the largest dipole moment and dipole polarizability.
Figure 5.2 sketches a comparison between HOMO/LUMO and elec-
trostatic descriptions of the H-bonded structure observed for
ð
:
19ea
0
2
2
Þ
2
. The
correspondence between the two descriptions is evident from the figure.
ð
HF
4
The dipolemoment
m
1
is a vector always directed along themain symmetry axis z, being positive
for A
d
B
þ d
with the heaviest atom A taken at the origin of the coordinate system.
