Solvation spectra. Part 50.—Spectrophotometric studies of the solvation of nitrate ions in protic and aprotic media

Abstract

Nitrate ions in aprotic solvents exhibit a single band at ∼1 353 cm^–1, in the absence of ion-pair formation. However, in protic solvents, (H₂O, MeOH)ν₃ is split into a doublet (∼1 345 and 1 400 cm^–1). This indication of descent in symmetry is interpreted in terms of relatively strong hydrogen-bonding to one or two oxygen atoms only. This effect, which contrasts with the behaviour of perchlorate ions, is interpreted in terms of the high polarisability of the nitrate ion. Application of simple equations for weak interactions in a trigonal-planar system suggests that a single strong hydrogen bond is favoured. This is supported by e.s.r. spectroscopic studies of irradiated aqueous and methanolic glasses containing nitrate ions, which show that the product nitrogen dioxide is not hydrogen bonded to the matrix. Addition of low concentrations of methanol to solutions of tetra-n-butylammonium nitrate in methylene chloride caused shifts and splittings of the ν₁ and ν₃ modes indistinguishable from those obtained with bulk methanol. This occured in parallel with changes in the O—H stretching mode indicative of strong 1:1 hydrogen-bonding between methanol and nitrate, thus supporting the concept of strong mono-solvation. Ion-pairing with sodium ions in aprotic solvents results in a comparable splitting of the ν₃ absorption.

Studies of the O—H stretching mode of interacting solvent molecules show that the strength of the hydrogen-bonds to nitrate are comparable to those formed by iodide ions and are far stronger than those formed by perchlorate ions.