CHEM01200604005 A. K. Pathak - Homi Bhabha National Institute

close vicinity of the halogen moiety. In case of hydrated clusters, X 2 .nH 2 O (X=Cl, Br &
I; n=1-8), the solvent stabilization and the interaction energies are very close for a few
small size clusters. But when the inter molecular hydrogen bond among water molecules
starts to build up; solvation energy surpasses the interaction energy. The variation of
solvent stabilization energy with size of the cluster (n) for all the systems is similar. On
the other hand, variation of the interaction energy with size of the cluster (n) is different
for Cl 2 system compared to that of Br 2 and I 2 system.
Table 4.1 indicates that the interaction energy decreases from I 2 .nH 2 O to
Cl 2 .nH 2 O systems for all sizes of the clusters except that for n=2. Thus the calculated
interaction energy in these halogen-water clusters should be able to predict the solubility
of X 2 in water. Calculated higher interaction energy for Br 2 .nH 2 O clusters compared to
that for Cl 2 .nH 2 O cluster is able to explain the higher solubility of bromine in bulk water
over chlorine. As I 2 .nH 2 O posses highest interaction energy among these three systems,
iodine in the gas phase is expected to have much higher solubility in the bulk water
compared to bromine and chlorine gases. The overall solubility order of gaseous halogens
in water is I 2 > Br 2 > Cl 2 .
4.4. Conclusions
Both ab initio (MP2) and hybrid exchange-correlation density functional method
have been applied to study microhydration of halogen gases with a split valence 6-
311++G(d,p) basis function. It is concluded that both Br 2 and I 2 exist as a charge
separated (Br +d -Br -d and I +d -I -d ) entity in the hydrated clusters. Though structures of
Cl 2 .nH 2 O clusters are similar to that of iodine system, Cl 2 does not exist as a charge
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