CHEM01200604005 A. K. Pathak - Homi Bhabha National Institute

clusters. Hydrated cluster having cyclic water network units, is the most stable conformer
for each size cluster (see Fig. 2.3). In each case, the distance between the C and O atoms
of carbonate moiety (CO 3•¯) is ~ 1.3 Å. The calculated distance between O (of CO 3•¯)
and H-bonded H atoms is 1.8-2.2 Å (SHB and DHB bond) and the distance between O
and H atoms in the inter water network is 1.9-2.1 Å (WHB bond).
2.3.1.3. Structure of CO 2 3 ⎯.nH 2 O Clusters
In the previous two sections, structures of singly anionic species are discussed. It
is also interesting to study structure of microhydrated polyatomic species with doubly
negative charge like carbonate anion (CO 2− 3 ). On full geometry optimization at
B3LYP/6-311++G(d.p) level of theory, different minimum energy structures are obtained
for each size of hydrated carbonate dianion cluster, CO 2− 3 .nH 2 O (n=1-6). It is observed
that the structures of conformers of each size of CO 2− 3 .nH 2 O cluster are very close to
those obtained for NO 3¯.nH 2 O and CO 3•¯.nH 2 O clusters. The most stable structure for
each size of hydrated cluster, CO 2− 3 .nH 2 O is displayed in Fig. 2.4. It is clear from the
figure that CO 2− 3 .nH 2 O clusters are stabilized by double hydrogen bonding (DHB), single
hydrogen bonding (SHB) as well as inter water hydrogen bonding (WHB). H-bonded
water network having two or three, H 2 O units are present in the different conformers of
these clusters. But structures with cyclic water network is not the most stable structures
for n=3-5 in contrast to the other hydrated clusters of Cl •− 2 , Br •− 2 , I •− 2 , CO •− 3 and NO − 3 .
For n=6, the most stable structure has one cyclic water network. No structure with four of
higher member water network is found. In each case, the distance between the C and O
atoms of the carbonate moiety (CO 2 3 ⎯) is ~ 1.3 Å. The calculated distance between O (of
47