(see Sects. 3.3.3, 3.4, [106–109] and references): Homonuclear I 2 has no permanent
dipole moment, but the I 2 (E) state has giant polarizability due to a huge I 2 ðE0
þ
g À
D0
þ
u Þ transition dipole moment l ED . The Lennard-Jones (6-12) potential
U R
ð Þ ¼ 4e R 0 =R
ð
Þ
12 À R 0 =R
ð
Þ
6
h
i
;
ð6:3:17Þ
R
is the I 2 (IP) center of mass– Rg distances,
e
is the potential depth,
R 0 is the R distances at which U(R) = 0,
representing the London dispersion forces, can be utilized to describe Rg– I 2 (IP)
vdW bonding in the T-shaped configuration. To estimate the C 6 ¼ 4eR
6
0 coefficient
of dispersion interaction, one takes into account the giant l ED = e∙r II , * 18∙10
−18
esu for low vibrational levels of the I 2 (E) state, and, hence, giant polarizability
a I 2 E
ð Þ % 2 Á 10
5 ˚
A
3 . In this case, the standard second-order expression for the C 6
coefficient can be simplified to
C 6 % a Rg l ED
j
j
2 :
ð6:3:18Þ
There is a correlation between a vdW complex dissociation energy D e and a C 6
dispersion coefficient
Fig. 6.32 Experimental binding energies of the RgI 2 (X, B and E) complexes, as functions of the
Rg polarizability (see Fig. 6.29 in [18])
240
6 Weakly-Bound Complexes and Clusters