| The nature in which an excess proton is accommodated in bulk water, H+(aq), and determination of the critical intermolecular interactions governing the unique properties of ionic liquids (IL) remain unresolved. Studies of solution-phase infrared spectroscopy have resulted in contentious debate on the microscopic structures giving rise to the bulk behavior of these systems. To circumvent the spectral broadening and complexity observed for the highly fluctuating ensemble in the solution phase, gas-phase spectroscopy of isolated, size-selected clusters provides a means of capturing the underlying molecular physics in a systematic, bottom-up approach. In this Dissertation, the vibrational spectra of protonated water clusters, H+(H2O)n ( n=2-28), and clusters of the prototypical IL 1-ethyl-3-methylimidazolium tetrafluoroborate, [EMIM+]n[BF 4--]n+/-1 ( n=0-3), are analyzed. The clusters were generated using electrospray ionization and "frozen" into well-defined arrangements through collisional cooling in a cryogenically cooled quadrupole ion trap, resulting in the acquisition of vibrational spectra directly comparable to those predicted by quantum-mechanical calculations. The evolution of spectral patterns with cluster size, in conjunction with theoretical predictions, do indeed clarify the key structural motifs and interactions present in the bulk systems. It is shown that the excess proton is captured as a surface-embedded hydronium ion, while electrostatics, and not hydrogen bonding, most influence the behavior of the prototype IL. |
