CHEM01200604005 A. K. Pathak - Homi Bhabha National Institute

Cluster experiments are carried out under NVE ensemble condition and the
precise value of the low temperature is not known. However, as these molecular clusters
are stable only in the low temperature, the experimental condition is such that the
temperature is kept below 200K. To choose a suitable method for the calculation of IR
spectra of Cl • 2 ⎯.nH 2 O clusters, weighted average IR spectra is generated at MP2, B3LYP
and BHHLYP level of theory at various temperature (T= 75 K to 200 K) with 6-
311++G(d,p) basis set. It is observed that the calculated IR spectrum at BHHLYP level of
theory at 100K is very close to the experimentally recorded spectrum. So, BHHLYP
functional is used to generate IR spectra for all the Cl • 2 ⎯.nH 2 O clusters. The minimum
energy structures (including the second hydration shell) of Cl • 2 ⎯.nH 2 O (n=1-5) clusters
are displayed in Fig. 3.2. Cl •− 2 .nH 2 O clusters are stabilized by double hydrogen bonding
(DHB), single hydrogen bonding (SHB) and inter water hydrogen bonding (WHB) as
discussed in Chapter 2. Due to these interactions, it is expected that IR bands due to
stretching (O-H) and bending modes of H 2 O in the hydrated clusters get shifted compared
to that of the free water molecule. For mono- and di-hydrated cluster, one and two
minimum energy structures have been found, respectively. The calculated scaled IR
spectra of Cl •− 2 .nH 2 O (n=1 and 2) cluster is also shown in Fig. 3.3. It is reported that for
mono- and di-hydrate cluster (n=1 and 2), the experimental IR spectra 24 corresponding to
the symmetric structure having one and two DHB, respectively (see Fig.3.2-1A and II-B).
First principles CPMD simulations are also carried out for 1.2 ps at 100K to
generate the dipole autocorrelation function (DACF) for Cl •− 2 .3H 2 O. The Fourier
transform to DACF (FT-DACF) is carried out to generate IR spectra. The IR spectrum
for Cl •− 2 .3H 2 O cluster based on FT-DACF is displayed in Fig.3.4 (I). It is clear from the
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