Use of a nitroxide radical probe to study the structure of aqueous and methanolic solutions of electrolytes

Abstract

Dilute aqueous and methanolic solutions of di-t-butyl nitroxide have been studied by e.s.r. spectroscopy, and changes in the ¹⁴N hyperfine coupling constant and in the line-widths have been measured as a function of a range of added electrolytes.

The results are discussed in terms of two equilibria, one involving the gain or loss of hydrogen bonds to the nitroxide oyxgen atom, and the other involving the formation of solvent shared units involving the cations, which result in changes in hydrogen-bond strengths. It is suggested that anions compete with R₂NO molecules for (OH) groups and hence cause a decrease in A(¹⁴N) whilst cations indirectly act as as a source of (OH) groups. This causes an increase in A(¹⁴N) for methanolic solutions, but not for aqueous solutions, since virtually all the nitroxide molecules are already hydrogen bonded. For many 1:1 electrolytes these two competing effects almost cancel. However, for tetra-alkylammonium salts the anion effect dominates, whilst for sodium tetraphenylboron the cation effect dominates. For doubly and trebly charged cations, units of the type [graphic omitted] are thought to cause an increase in the strength of the hydrogen bonds, and a consequent extra increase in A(¹⁴N). The results are discussed together with recent near infrared data for the same solutions.

Line-width trends follow changes in A(¹⁴N) as previously observed for R₂NO molecules in aqueous solvent mixtures. Differential line-broadening was observed at low temperatures being most marked for aqueous solutions of sodium tetraphenylboride. In contrast, methanolic solutions of this salt were quite normal. This anomalous result is discussed in terms of charge-transfer complex and clathrate cage formation.