Abstract

The ionization of hydrogen halide acids HX (X=Cl, Br) in the presence of water in the ground and excited electronic states is an important fundamental process, with possible implications for the ozone layer depletion. Experimental investigations of acid dissociation and hydration of ion pairs [X - (H 2 O) n H 3 O + ] at the molecular level are hindered by instrument limitations, and to date, theoretical studies address the mechanism of acid ionization in polar and non-polar solvents only partially. This thesis aims at characterizing ground-state acid-water complexation and ionization, and possible effects of water on the ionization of HX in electronically excited states. As for ground-state ionization, the structures of HCl and HBr in aqueous clusters of different sizes were characterized with second-order Møller-Plesset perturbation theory (MP2), and coupled cluster theory with single, double and perturbative triple excitations [CCSD(T)], paying particular attention to the critical number of water molecules needed for acid ionization to occur. Combined results from the quantum theory of atoms in molecules (AIM) and infrared (IR) vibrational spectra analyses are used to unambiguously characterize the nature of bonding in acid-water clusters. Global minimum-energy cluster structures suggest that both HCl and HBr form ionized complexes in the presence of four water molecules. As for possible excited-state ionization of hydrogen halides, the excited-state electronic structures and potential energy curves of gas-phase HBr were first obtained from multi-reference configuration interaction (MRCI) calculations. Investigation of the influence of water molecules on the excited-state potential energy curves of HBr then revealed the feasibility of excited-state ionization, and thus photo-ionization in water clusters, due to opening of an ion-pair dissociation channel. This work thus claims the interpretation of recent experimental findings [Science, 2002], in which HBr photo-ionization was observed in small water clusters, but not for clusters larger than size 4, presumably because of ground-state ionization that quenches the HBr photo-ionization channel.