CHEM01200604005 A. K. Pathak - Homi Bhabha National Institute

solvation energy for larger hydrated clusters. The slope of the best-fitted linear equation
suggests that the solvation energy per solvent water unit is 11.25 kcal/mol. In other
words, on an average each additional H-bond provides a solvent stabilzation of 5.6
kcal/mol at large n limit, assuming each additional solvent water molecule increases the
number of H-bonds by two. However, this assumption is valid for very large size (n) of
cluster. It is worthwhile to mention that basis set superposition error (BSSE) calculated
for mono- and di- hydrated clusters following counterpoise method 45 is less than 10%.
It is to be noted that the stabilzation energy for the hydrated cluster, I •− 2 .nH 2 O
calculated by discrete model corresponds to the internal energy of the molecular clusters.
The stabilzation energy of the clusters ( E w
solv
or E solv ) naturally increases with the
increase in the number of solvent molecules accounting ion-solvent interaction as well as
inter water H-bonding interaction. However, stabilzation energy has the status of free
energy in case of the continuum model. One can see form Fig. 2.5 that stabilzation energy
profiles do not indicate closing of any geometrical shell. Thus, it does not provide any
information on the hydration number of I 2 •− .
To estimate the energy of interaction between the solute dimer radical anion, I 2
•−
and the water cluster, (H 2 O) n , a quantity, interaction energy (E int ) in I 2 •− .nH 2 O clusters
can be defined as
int
E = E − ( E + E ),
where, E I
−
I . nH O ( HO)
n I
. . −
2 2 2 2
. −
2 2
. nH O
, E( HOn 2 )
and E I
.− refer to the energy of the cluster I •− 2 .nH 2 O, the energy
2
of (H 2 O) n system and the energy of I 2 •− system, respectively. The energy of the (H 2 O) n
system is calculated by removing iodine part from the optimized geometry of that
50