Knowledge Bank

The antisymmetric stretching mode of the nitrate ion $(\nu 3)$ has been used as a probe for the solvation of $M+NO3\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$ 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 $\sim 200cm-1$ 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 $M+NO3\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$ (solvent). 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.