Characterization of Solvent Effects in Microhydrated Ionic Clusters using Cryogenic Vibrational Spectroscopy

摘要

The molecular level interactions between ionic and neutral species give rise to the macroscopic chemical and physical properties of bulk electrolyte systems. A fundamental knowledge of these interactions facilitates understanding of this behavior and enables rational manipulation of the system at the molecular level. Infrared spectroscopy reveals the vibrational fingerprints that are characteristic of the chemical environments of bonds, making it a powerful tool for characterization of local structure. Standard solution phase and surface-sensitive spectroscopies, however, suffer from the drawback that the spectra contain simultaneous contributions from species in many distinct chemical environments, which also undergo thermal fluctuations over time. Gas-phase vibrational spectroscopy of cryogenically (~l0 K) cooled, mass-selected clusters represents an alternative, "bottom-up" approach for elucidating the underlying chemical physics with a high degree of experimental control over the precise chemical composition and temperature. In this Dissertation, the infrared spectra of ionic clusters generated by electrospray ionization (ESI) and cooled in radiofrequency ion traps are collected in a "messenger-tagging" action spectroscopic scheme referred to as cryogenic ion vibrational predissociation (CIVP). The perturbations induced by the weakly bound molecular adducts (He, H2, N2, etc.) necessary for the acquisition of CIVP spectra are assessed by their effect on the spectrum of the NH4+(H 2O) cation-hydrate. Alternative approaches for acquiring linear action spectra based on multiple laser resonance spectroscopy are demonstrated for the I-(H2O)2 complex and its isotopologues. Gas phase reactions of dinitrogen pentoxide (N2O5) with hydrated halide ions (X = CI, Br, I) are examined, and the observed [XN 2O5]- species are identified as exit channel ion-molecule complexes in the formation of XNO2, which has implications for modeling the atmospheric chemistry of nitrogen oxides. The effects of solvation on ions and ion pairs/complexes can be rigorously studied by forming clusters containing a precise number of solvent molecules around the solute ion. The modulation of the ion-ion and ion-solvent interactions in the binary [M2+CD3CD2COO- ] + (M = Mg, Ca) and ternary [MgSO4Mg]2+ ionic complexes through the first hydration shell are established. In many cases, the clusters can be made sufficiently large that their behavior becomes close to that of the bulk system. This enables a compelling way to extrapolate cluster spectra to understand that of the interfacial or aqueous environment using electronic structure theory and molecular dynamics simulations.