CHEM01200604005 A. K. Pathak - Homi Bhabha National Institute

On full geometry optimization of NO 3
− .8H2 O cluster, twelve minimum energy
structures are predicted and the final optimized structures are displayed in Fig. 2.2-
VIII(A-L). None of these structures yields imaginary frequency on Hessian calculations
suggesting these minimum energy configurations as true equilibrium structures. As in
case of the hepta- hydrated cluster, the most stable structure has a cyclic five member
water network arrangement keeping the solute anion at the surface. Similar geometrical
arrangements, keeping the solute at the surface, are also observed in the remaining of
structures except for the structures VIII-H and VIII-K. In these two cases, NO − 3 is
trapped inside H-bonded solvent water networks.
In all the minimum energy configurations of NO 3
− .nH2 O cluster, NO bond
distance is calculated in the range of 1.25-1.27 Å. The calculated distance between O of
NO 3
− and H-bonded H atoms is 1.8-2.3 Å and the distance between O and H atoms in
inter water network (WHB) is 1.8-2.0 Å. In all the structures, the calculated SHB distance
is shorter than that of DHB distance. Structure having cyclic inter water H-bonding
network is more stable over other structures in a particular size of hydrated cluster. It is
observed that more than three solvent water molecules cannot reside in a DHB
arrangement in these hydrated clusters. It is also observed that more than five water
molecules cannot stay in the cyclic water network. Most of the minimum energy
configurations in NO 3
− .nH2 O cluster show surface structure in which the solute resides at
the surface of the solvent water network. In case of large size hydrated clusters,
38