CHEM01200604005 A. K. Pathak - Homi Bhabha National Institute

cluster. Nine conformers are obtained with all real frequencies based on full geometry
optimization followed by Hessian calculations. The optimized structures are displayed in
Fig. 2.1-VIII(A-I). One can see from Fig. 2.1-VIIIA that the anion moiety is sandwiched
between the two cyclic water tetramers in the most stable conformer. Structure VIII-B
making one cyclic water tetramer is less stable than the most stable conformer by 0.67
kcal/mol. Structure VIII (A-C, E-F) has at least one cyclic water tetramer. The least
stable conformer (~10 kcal/mol less stable compared to VIII-A), (see Fig. 2.1-VIII(I)) has
two H 2 O cyclic trimers and two DHB arrangements.
It is noticed that several minimum energy conformations with a small energy
difference can exist for the hydrated cluster, I •− 2 .nH2O (n=3-8). It is clear from the figure
that the I •− 2 .nH 2 O clusters are stabilized by double hydrogen bonding (DHB), single
hydrogen bonding (SHB) and inter water hydrogen bonding (WHB). Conformation
having H-bonded water network (WHB arrangements) are more stable over the other
structures where H 2 O units are connected to the anion moiety independently either by
SHB or DHB for a particular size of the cluster. H-bonded water network, having two,
three or four H 2 O units, are present in different conformers of these clusters. Hydrated
cluster having cyclic water network units is the most stable conformer for each size
cluster. Hydrated I •− 2 .nH 2 O clusters of size n=4-8 contain at least one four member water
ring (see Fig. 2.1). In each case, the distance between the two I atoms is ~ 3.3 Å, the
distance between I and H-bonded H atoms is 2.3-2.8 Å (SHB and DHB bond) and the
distance between O and H atoms in inter water network is 1.9-2.1 Å (WHB bond).
Structures of microhydrated species, Cl •− 2 .nH 2 O and Br •− 2 .nH 2 O are also
calculated. Similar minimum energy structures are also observed for Cl •− 2 .nH 2 O and
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