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Title:The investigation of solvent shell structure and reactivity in solvated metal ion clusters
Author(s):Selegue, Thomas James
Doctoral Committee Chair(s):Lisy, James M.
Department / Program:Chemistry
Degree Granting Institution:University of Illinois at Urbana-Champaign
Subject(s):Chemistry, Physical
Abstract:The structure and reactivity of gas phase ionic clusters were studied using molecular beam mass-spectrometry, vibrational predissociation spectroscopy and Monte Carlo simulation. Cluster ions of the form M$\sp+$(S)$\sb{\rm N}$, M = Cs, Na and S = CH$\sb3$OH, NH$\sb3$, CH$\sb3$NH$\sb2$, (CH$\sb3)\sb2$CO and (CH$\sb3)\sb2$SO were prepared in the source chamber of continuous molecular beam apparatus. Quasi-stable cluster ions were mass selected with a quadrupole mass filter and size selective infrared spectroscopy was carried out on size selected clusters with as many as thirty solvent molecules. Vibrational predissociation of the clusters using a line tunable CO$\sb2$ laser was detected using the beam depletion method. By comparing the infrared spectra of the cluster ions as they sequentially increased in size, information on the solvent shell structure about the central metal ion was gained. For the sodium ion, a first solvent shell size of six methanol molecules and six ammonia molecules was determined. For the cesium ion, ten molecules of methanol and ammonia filled the first solvent shell. Insensitivity of the vibrational mode that was probed in CH$\sb3$NH$\sb2$ and (CH$\sb3)\sb2$SO to cluster structure made determination of the solvent shell structure experimentally uncertain. The clusters M$\sp+$((CH$\sb3)\sb2$CO)$\sb{\rm N}$ do not absorb in the region of the infrared accessible with the CO$\sb2$ laser. Competitive solvation of an ion in a binary solvent environment was studied by preparing mixed solvent clusters in which only one solvent was an infrared chromophore: M$\sp+$((CH$\sb3)\sb2$CO)$\sb{\rm N}$(CH$\sb3$OH)$\sb{\rm M}$. Local structure of the solvent in the vicinity of the ion was found to be determined by both the electrostatic influence of the ion and hydrogen bonding between molecules. Monte Carlo simulations were carried out to help aid in the interpretation of the vibrational spectra. Pairwise-additive potentials were used to model the interaction between the molecules and the ion. A Metropolis algorithm was used to minimize the energy of the system using simulated annealing. Temperatures for the simulation were chosen by assuming either RRK or RRKM dissociation rates and casting the clusters in an evaporative ensemble. The results of the simulations compared quite favorably with the experiments in most of the systems studied. A wealth of structural information was also gained from the theoretical studies, including the relative importance of the ion-molecule and molecule-molecule interactions and the role of hydrogen bonding in structuring the cluster ion. Reactivity of the molecules solvating the ion was studied in select systems. Product mass peaks were detected in mass spectra of M$\sp+$(S)$\sb{\rm N}$ and positively identified using isotopic substitution. The dehydration of methanol to form dimethyl ether and water is driven within M$\sp+$(CH$\sb3$OH)$\sb{\rm N}$ by the excess energy deposited in the cluster during formation. A strong dependence of reactivity on cluster size was observed and correlated well with solvation shell fillings. A mechanism for this fundamental process is suggested. The methods described in this thesis comprise one of the most sensitive probes of solvation on a microscopic scale developed to date.
Issue Date:1994
Rights Information:Copyright 1994 Selegue, Thomas James
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9416431
OCLC Identifier:(UMI)AAI9416431

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