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INFRARED STUDIES OF SOLVATION OF MATRIX ISOLATED SIMPLE ION PAIRS BY APROTIC SOLVENTS IN MIXED MATRICES

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Title: INFRARED STUDIES OF SOLVATION OF MATRIX ISOLATED SIMPLE ION PAIRS BY APROTIC SOLVENTS IN MIXED MATRICES
Creators: Toth, John P.; Devlin, J. Paul
Issue Date: 1979
Abstract: The antisymmetric stretching mode of the nitrate ion $(\nu_{3})$ has been used as a probe for the solvation of $M^{+}NO_{3}{^{-}}$ ion-pairs by the aprotic solvents acetonitrile, THF, DMSO and DMF. In the infrared spectra of the vapors of these salts isolated in a pure argon matrix at 12 K, the doubly-degenerate $\nu_{3}$ mode is split $\sim 200 cm^{-1}$ into two components $\nu_{3a}$ and $\nu_{3b}$, with a separation $\Delta \nu_{3}$ that is cation dependent. This is indicative of the presence of ion-pairs. As one increases the percentage of aprotic solvent in an argon matrix, the magnitude of $\Delta \nu_{3}$ decreases in a stepwise fashion, indicating a reduction in the perturbation of the nitrate ion by the cation as the latter is solvated. In the extreme case of a 100% solvent matrix, the matrix spectra closely resemble the spectra of the corresponding saturated liquid solutions. The exception to this is DMSO, whose liquid solution spectra show a single $\nu_{3}$ band, indicative of ion-pair dissociation. In most cases, the spectra obtained in matrices of intermediate solvent concentration show resolved $\nu_{3a}-\nu_{3b}$ pairs which are assigned to individual solvation steps, i.e., to the species $M^{+}NO_{3}{^{-}}$ (solvent)$_{n}$. Such assignments for the complete range of solvent concentrations permit the determination of the cation solvation number of a given ion pair in a given solvent. The magnitude of the reduction of $\Delta \nu_{3}$ per solvent molecule is discussed in terms of solvent properties and possible solvation mechanisms.
URI: http://hdl.handle.net/1811/11118
Other Identifiers: 1979-WD-07
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