CHEM01200604005 A. K. Pathak - Homi Bhabha National Institute
CHEM01200604005 A. K. Pathak - Homi Bhabha National Institute
CHEM01200604005 A. K. Pathak - Homi Bhabha National Institute
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geometrical parameters close to MP2 values. So, BHHLYP functional is used for<br />
geometry search throughout.<br />
Several minimum energy structures are obtained for I •− 2 .nH 2 O (n=1-8) clusters,<br />
on full geometry optimization. All the stable conformers for each size hydrated cluster<br />
are displayed in Fig. 2.1. Fig. 2.1-I shows the structure of monohydrate cluster (I •− 2 .H 2 O)<br />
consisting symmetrical double hydrogen bonding (DHB) arrangement where two I atoms<br />
and two H-atoms of the H 2 O molecule are connected by H-bonding of distance ~2 Å. It is<br />
worthmentioning that all possible combinations of initial geometry for optimization<br />
converged on the same structure. Two minimum energy structures are obtained for the dihydrate<br />
(I •− 2 .2H 2 O) cluster. Structure II-A having one DHB, one single hydrogen bonding<br />
(SHB) and one inter water H-bonding (WHB) arrangements is calculated to be more<br />
stable compared to the structure II-B having two symmetrical DHB arrangements by 1.42<br />
kcal/mol. It appears that structure II-A is formed when H-bonded water dimer is<br />
connected to two I atoms of I •− 2 system by three free H atoms via H-bonding. But the<br />
initial guess structure of II-A is the one where two water molecules are connected via one<br />
WHB arrangement and two SHB arrangements. II-B is obtained from the initial guess<br />
where a water molecule is added to the structure I from opposite side of H 2 O making<br />
another DHB arrangement. It is worth mentioning that structure II-B is also obtained<br />
from a initial guess where two water molecules approach the I •− 2 moiety from two sides<br />
of I-I axis. Structure II-B is planar having the dihedral angle, O-I-I-O as 180 o . The energy<br />
for a WHB and SHB in such system is calculated to be 7.0 and 6.6 kcal/mol, respectively<br />
whereas the H-bonding energy in neutral water dimer is ~6.0 kcal/mol. Thus, a particular<br />
minimum energy structure is obtained on compromise between the formation of inter<br />
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