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Title:Infrared spectroscopy and Monte Carlo simulations of gas phase, solvated alkali ions
Author(s):Draves, Jeffrey Albert
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 solvation of the alkali ions Na$\sp{+}$ and Cs$\sp{+}$ by methanol has been investigated by gas-phase vibrational spectroscopy and Monte Carlo simulations of small ion clusters: Na(CH$\sb3$OH) $\sbsp{\rm N}{+}$, N = 6 to 15 and Cs(CH$\sb3$OH) $\sbsp{\rm N}{+}$, N = 4 to 25. The solvated ions, generated by combining a thermionic alkali ion source with a molecular beam source, have considerable amounts of internal energy. The excess energy is dissipated by evaporation. The quasi-stable cluster ions are mass-selected for vibrational predissociation spectroscopy. The spectra are collected using a line-tunable cw-CO$\sb2$ laser.
Vibrational spectra indicate that the first solvation shell about the Na$\sp{+}$ consists of six methanols while that about the Cs$\sp{+}$ consists of ten. Large clusters of Cs(CH$\sb3$OH) $\sbsp{\rm N}{+}$ (N $\geq$ 18) appear to have small clusters of methanol bound to the surface of the solvated ion. Na(CH$\sb3$OH) $\sbsp{\rm N}{+}$ clusters appear to have two structurally enhanced regions, i.e. a first and a second solvation shell.
Monte Carlo simulations using pairwise interaction potentials were run at temperatures between 200 K and 300 K for the solvated ions Na(CH$\sb3$OH) $\sbsp{\rm N}{+}$, N = 6-8, 10, 12, 15, and 25, and for Cs(CH$\sb3$OH) $\sbsp{\rm N}{+}$, N = 6-16, and 25. Simulation results show the first solvation shell of Na$\sp{+}$ and Cs$\sp{+}$ is filled by 6.2 and 10.5 methanols, respectively. These numbers are consistent with experimental results. The simulations also indicate that hydrogen bonding plays a significant role in shaping the solvent shell structure for both species.
Once the first solvation shell is filled, the size of the solvent shell appears to be independent of additional solvent molecules. After N = 6 for Na(CH$\sb3$OH) $\sbsp{\rm N}{+}$, and N = 10 for Cs(CH$\sb3$OH) $\sbsp{\rm N}{+}$ methanols begin entering the second solvent region, this shell is not filled before methanols enter the third solvent region. This is an indication of the extent to which the ion influences the solvating methanols.
Gas-phase solvated ions appear to be useful models for dilute electolyte solutions. This technique produces solvated ions which exhibit several solvent regions. With the combination of the Monte Carlo simulations and vibrational spectroscopy, a clear picture of the solvation shell structure can be obtained.
Issue Date:1990
Rights Information:Copyright 1990 Draves, Jeffrey Albert
Date Available in IDEALS:2011-05-07
Identifier in Online Catalog:AAI9114225
OCLC Identifier:(UMI)AAI9114225

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