Microhydration shell structure in Cl2-.nH2O clusters: A theoretical study

We present the results of a detailed study on structure and electronic properties of hydrated cluster Cl2*-.nH2O (n = 1-7) based on a nonlocal density functional, namely, Becke's [J. Chem. Phys. 98, 1372 (1993)] half and half hybrid exchange-correlation functional with a split valence 6-311++G(d,p) basis function. Geometry optimizations for all the clusters are carried out with various possible initial guess structures without any symmetry restriction. Several minimum energy structures (conformers) are predicted with a small difference in total energy. There is a competition between the binding of solvent H2O units with Cl2*- dimer radical anion directly through ion-molecule interaction and forming interwater hydrogen-bonding network in Cl2*-.nH2O (n > or = 2) hydrated cluster. Structure having interwater H-bonded network is more stable over the structure where H2O units are connected to the solute dimer radical anion Cl2*- rather independently either by single or double H bonding in a particular size (n) of hydrated cluster Cl2*-.nH2O. At the maximum four solvent H2O units reside in interwater H-bonding network present in these hydrated clusters. It is observed that up to six H2O units are independently linked to the anion having four double H bondings and two single H bondings suggesting the primary hydration number of Cl2*- to be 6. In all these clusters, the odd electron is found to be mostly localized over the two Cl atoms and these two atoms are bound by a three-electron hemibond. Calculated interaction (between solute and different water clusters) and vertical detachment energy profiles show saturation at n = 6 in the hydrated cluster Cl2*-.nH2O (n = 1-7). However, calculated solvation energy increases with the increase in number of solvent H2O molecules in the cluster. Interaction energy varies linearly with vertical detachment energy for the hydrated clusters Cl2*-.nH2O (n < or = 6). Calculation of the vibration frequencies show that the formation of Cl2*(-)-water clusters induces significant shifts from the normal stretching modes of isolated water. A clear difference in the pattern of IR spectra is observed in the O-H stretching region of water from hexa- to heptahydrated cluster.