Identification of major solution equilibria from matrix-solvation spectra

Abstract

The simple ion pairs that exist in the vapour phase of the anhydrous oxyanion salts near their respective melting points can be readily trapped as the isolated bare ion pair in inert non-solvating matrices. The inclusion of a common solvent (S) as a third component of a matrix sample allows the selective formation of the ion-pair solvates S_n·M⁺NO^–₃. When the ion pair is Li⁺NO^–₃ the infrared spectrum of the nitrate ν₃(e) mode is exceptionally sensitive to the value of ‘n’, which can be varied from 0 to n_max, corresponding to completion of the coordination shell of the cation of the ion pair. In recent years the individual lithium nitrate ion-pair solvates have been characterized for a number of different solvents and for the full range of n values. For non-dissociating solvents such as acetone and tetrahydrofuran, the spectra for the fully solvated ion pairs, S_n·Li⁺NO^–₃, for which n=n_max, may closely resemble the spectra for the salt dissolved in the corresponding liquid solvent. However, such liquid solution spectra are quite sensitive to the sample temperature, as variations of ca. 100 K regularly cause changes that correspond to a unit change in the solvation number. In addition to confirming the existence of the solvates as distinct spectroscopic species, the matrixsolvation spectra for a given solvent are informative of the liquid–solution solvation number, the temperature variation of that number and, indirectly, the thermodynamics of the transfer of solvent from the bulk liquid solution to the cation coordination shell. New data for acetone and t-butyl alcohol show that this transfer occurs exothermically with a magnitude similar to that found previously for tetrahydrofuran and other ether solvents (ca. 2 kcal). For acetone, for example, the favoured 3-solvate at 318 K, (acetone)₃·LiNO₃, is replaced by (acetone)₄·LiNO₃ at 173 K.