| When a charged solute ion is introduced into liquid water, the solvation free energy is typically on the order of several electron volts (eVs), which qualitatively is understood in the context of the simple Born solvation model based on a continuum dielectric ansatz. The breakdown of this treatment at the quantitative level is due to the fact that, very close to the ion, the solvent molecules adopt well-defined structures to the extent that the molecular nature of the ion-solvent interactions becomes important. To explore how these effects become manifest in the structure of the surrounding solvation network, cryogenic processing of isolated, size-selected ionic clusters present a promising new way to study ion hydration and establish these structures using vibrational spectroscopy.;The emerging method of cryogenic ion vibrational predissociation (CIVP) only has only been applied to a few prototype systems (simple peptides, catalyst precursors, protonated water clusters, etc.), and, while the information content provided by this technique varies widely with different systems, the general advantages clearly are established. First, the solvent network surrounding a solute-ion system can be frozen into local minima of the potential-energy surface close to its vibrational zero-point level, which yields well-defined structural signatures in its infrared spectrum. Furthermore, the extreme sensitivity of the vibrational energy-level structure to subtle changes in the local environment of each solvent molecule is exploited to trace distinct IR features that originate from individual sites within the H-bonded network of water molecules surrounding the ion. Second, the excess charge, which is essential for this technique, allows for isolation of the solvent shell around a single atomic or molecular ion without the strong perturbative effects due to the presence of counterions. Lastly, IR spectroscopy coupled to mass spectrometry offers a unique opportunity to trace the structural evolution of the solvation shell as individual water molecules are added stepwise to the ionic assembly. The resulting vibrational spectra can be analyzed effectively with ab initio electronic structure calculations to yield a detailed molecular-level understanding of the physics at play when the Born continuum description for ion solvation fails at the very onset of hydration.;CIVP spectroscopy allows study of the solvation mechanisms and the shapes of a solvent network around virtually any ion, and this thesis presents three prototypical systems, each chosen to explore an archetypal behavior common to an entire class of solute ions. Expanding the scope beyond the typical search for conformers, each chapter exploits the peculiarities of a particular system to develop novel experimental capabilities that yield new information about the behavior of a wide range of systems. These techniques include spectral isolation of the vibrational features originating from a single H2O molecule within a complex network, the onset of large-amplitude motion when the internal energy of a system is increased sequentially, and adaptation of the trace isotope scheme, common in condensed-phase spectroscopy of aqueous systems, to follow the microscopic mechanics of spectral diffusion. All these examples serve to highlight the diverse array of phenomena that can be addressed when cryogenically processed, mass-selected ions are studied by IR spectroscopy. |
