CHEM01200604005 A. K. Pathak - Homi Bhabha National Institute

generate IR spectra. Such study should help to enhance our understanding on molecular
level interactions between solvent water molecules and negatively charged ions in the
aqueous solution. The IR studies on size selected hydrated anion clusters also play a
critical role to follow the evolution of molecular properties with the number of solvent
water molecules present in the cluster and to bridge the gap between the monohydrated
cluster to the ion (I⎯) in bulk aqueous solution, I⎯(aq). Recently IR spectra of Cl • 2 ⎯.nH 2 O
(n=1-5) is reported based on size selected cluster spectroscopy by Johnson and
coworkers. 24 In this chapter, theoretical IR spectra of Cl • 2 ⎯.nH 2 O are reported in detail
and compared with the experimental ones. Weighted average IR spectra is calculated
based on the statistical population of different minimum energy configurations of a
particular size hydrated cluster at 100 K. Fourier transform to the dipole auto-correlation
function is also carried out to include dynamical contribution to the calculated IR spectra.
3.2. Theoretical Approach
Vibrational analysis is performed at the same level of theory as for geometry
search discussed in chapter 2 to generate IR spectra following harmonic approximations.
Frequency of different normal modes of a few small hydrated clusters is also calculated
by taking anharmonic nature of vibration using vibrational self-consistent field (VSCF)
method. With the increase in number of solvent water molecules in hydrated clusters, the
number of minimum energy configurations with close in energy for hydrated clusters of
each size is expected to increase. Experimental IR spectrum of particular size of cluster
includes contribution from all possible minimum energy structures, which are survived at
that particular experimental condition. Thus weighted average properties of clusters
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