Chemistry Reference
In-Depth Information
x
χ
B
χ
A
θ
B
θ
A
R
z
ϕ
y
A
B
Figure 11.4 The five angles specifying in general the relative orientation of two
polyatomic molecules
(
x
B
) shown in Figure 11.4. The first three angles describe the
orientation of the principal symmetry axes of the two molecules and the
latter two the rotation about these axes.
In what follows, we shall limit ourselves mostly to consideration of the
long-range dispersion interaction between (i) two linear molecules A and
B (top of Figure 11.5) and (ii) an atomA, at the origin of the intermolecular
coordinate system, and a linear molecule B, whose orientation with
respect to the
u
A
,
u
B
,
w
,
x
A
,
z-axis is specified by the single angle
u
(bottom of
Figure 11.5).
The linear molecule has two dipole polarizabilities,
a
jj
, the parallel or
longitudinal component directed along the intermolecular axis, and
a
?
,
the perpendicular or transverse component perpendicular to the inter-
molecular axis (McLean and Yoshimine, 1967a). The molecular isotropic
polarizability can be compared to that of atoms, and is defined as
a ¼
a
jj
þ
2
a
?
ð
11
:
40
Þ
3
while
jj
?
Da ¼ a
a
ð
11
:
41
Þ
jj
.
The composite system of two different linear molecules hence has four
independent elementary dipole dispersion constants, which in London
?
is the polarizability anisotropy, which is zero for
a
¼ a
