Knowledge Bank

The vibrational spectra of bare ion pairs, such as $Li+NO3\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$, accurately reflect the magnitude of the cation distortion of the anion through the splitting of otherwise degenerate modes. These large splittings ($262cm-1$ for $Li+NO3\mathrm{<mrow\; data-mjx-texclass="ORD">-</mrow>}$ in an argon matrix) also provide a useful probe of the effect of cation solvation on the ion pairs since complete solvation, as occurs in liquid solutions of non-dissociating solvents, results in near-collapse of the splitting. The effect of individual solvation steps on the ion pairs can be studied by the measurement of infrared spectra for species such as $S1\cdot LiNO3,S2\cdot LiNO3$ etc. prepared in inert gas matrices containing an appropriate fraction of the solvent, S. High quality FT-IR spectra allow the evaluation of reliable cation solvation numbers, an insight to the nature of the anion-solvent interaction, and an analysis of interplay between cation-solvent bonding and steric forces within the cation solvation shell.