Anomalously strong effect of the ion sign on the thermochemistry of hydrogen bonded aqueous clusters of identical chemical composition.

The sign preference of hydrogen bonded aqueous ionic clusters X+/-(H(2)O)(i) (n =1-5, X = F; Cl; Br) has been investigated using the Density Functional Theory and ab initio MP2 method. The present study indicates the anomalously large difference in formation free energies between cations and anions of identical chemical composition. The effect of vibrational anharmonicity on stepwise Gibbs free energy changes has been investigated, and possible uncertainties associated with the harmonic treatment of vibrational spectra have been discussed.

Quantum nature of the sign preference in ion-induced nucleation.

Observed first in Wilson's pioneering experiments in the cloud chamber, the sign preference has remained a mystery for more than a century. We investigate the sign preference using a quantum approach and show that this puzzling phenomenon is essentially quantum in nature. It is shown that the effect of the chemical identity of the core ion is controlled by the electronic structure of the core ion through the influence on the intermolecular bonding energies during the initial steps of cluster formation. Our results demonstrate the superiority of the quantum approach and indicate fundamental problems of conventional ion-induced nucleation theories, in which the electronic structure of the core ion is either ignored or not treated rigorously.