Vibrational spectroscopy provides the most widely used means for characterizing ion associates. Both infrared spectroscopy and Raman spectroscopy have been used. Anions containing a CN group, such as cyanide, cyanate and thiocyanide have a vibration frequency a little above 2000 cm, which can be easily observed, as the spectra of most solvents (other than nitriles) are weak in this region. The anion vibration frequency is "shifted" on formation of ion pairs and other associates, and the extent of the shift gives information about the nature of the species. Other monovalent anions that have been studied include nitrate, nitrite and azide. Ion pairs of monatomic anions, such as halide ions, cannot be studied by this technique. Standard NMR spectroscopy is not very useful, as association/dissociation reactions tend to be fast on the NMR time scale, giving time-averaged signals of the cation and/or anion. However, diffusion ordered spectroscopy (DOSY), with which the sample tube is not spinning, can be used as ion pairs diffuse more slowly than do single ions due to their greater size.
Nearly the same shift of vibration frequency is observed for solvent-shared ion pairs of LiCN, Be(CN) and Al(CN) in liquid ammonia. The extent of this type of ion pairing decreases as the size of the cation increases. Thus, solvent-shared ion pairs are characterized by a rather small shift of vibration frequency with respect to the "free" solvated anion, and the value of the shift is not strongly dependent on the nature of the cation. The shift for contact ion pairs is, by contrast, strongly dependent on the nature of the cation and decreases linearly with the ratio of the cations charge to the squared radius:
:Cs > Rb > K > Na > Li;
:Ba > Sr > Ca.
The extent of contact ion pairing can be estimated from the relative intensities of the bands due to the ion pair and free ion. It is greater with the larger cations. This is counter to the trend expected if coulombic energy were the determining factor. Instead, the formation of a contact ion pair is seen to depend more on the energy needed to displace a solvent molecule from the primary solvation sphere of the cation. This energy decreases with the size of the cation, making ion pairing occur to a greater extent with the larger cations. The trend may be different in other solvents.
Higher ion aggregates, sometimes triples MXM, sometimes dimers of ion pairs (MX), or even larger species can be identified in the Raman spectra of some liquid-ammonia solutions of Na salts by the presence of bands that cannot be attributed to either contact- or solvent-shared ion pairs.
Evidence for the existence of fully solvated ion pairs in solution is mostly indirect, as the spectroscopic properties of such ion pairs are indistinguishable from those of the individual ions. Much of the evidence is based on the interpretation of conductivity measurements.