Dynamics of Hydrogen Bromide Dissolution in the Ground and Excited States

Abstract

The dissolution of acids is one of the most fundamental solvation processes, and an important issue is the nature of the hydration complex resulting in ion pair formation. We used femtosecond pump-probe spectroscopy to show that five water molecules are necessary for complete dissolution of a hydrogen bromide molecule to form the contact ion pair H+.Br-(H2O)n in the electronic ground state. In smaller mixed clusters (n < 5), the ion pair formation can be photoinduced by electronic excitation.

Clusters: A bridge across the disciplines of physics and chemistry

“…A recent investigation of the dynamics of HBr dissolution in mixed clusters with water (111,112) has finally provided an answer for this system. Hydrated complexes formed by interacting water with clusters of HBr were subjected to femtosecond pump-probe spectroscopic investigation.…”

“…Some spectroscopic evidence (111,112,115) for the interaction of four waters with HCl leading to ion-pair formation comes from matrix isolation studies, while related spectroscopic information for smaller water clusters in the gas phase is given in ref. 116.…”

Electric Dipole Moments of Nitric Acid-Water Complexes Measured by Cluster Beam Deflection

“…It is an interesting problem to study the onset of this process in water clusters. For example, the minimum number of water molecules required to promote dissolution of HBr was determined experimentally to be five [1,2] and more recently molecules of HCl were observed to dissociate in the presence of only four water molecules even in the ultra-cold environment of a helium nanodroplet [3,4]. The electric dipole moment of solvated charge-separated species is expected to be higher than that of the intact molecule [5].…”

Electric Dipole Moments of Nanosolvated Acid Molecules in Water Clusters

“…According to calculations, the acidically dissociated structure is supported starting from n=4; most of the theoretical approaches agree that at this size the dissociated form represents the global minimum on the potential energy surface [18][19][20][21][22][23][24]. Experimentally, for neutral clusters the question has been probed by laser spectroscopy (see, e.g., [25][26][27][28]) but finding a conclusive spectroscopic signature of the onset of dissociation in a nanocluster is not straightforward.As a matter of fact, the challenge is not just experimental but conceptual. The free water clusters in natural and laboratory environments generally exist at temperatures appreciably above absolute zero [29].…”