Solvation spectra. Part 31.—Electron spin resonance studies of ion-pair formation between nitrobenzene anions and sodium or t-butylammonium cations

Abstract

The ¹⁴N hyperfine coupling in the anions of mononitrobenzene and m- or p-dinitrobenzenes is sensitive to environment. In methanol or ethanol the coupling constants are large because hydrogen bonding strongly perturbs the structure. Addition of sodium or lithium salts results in the formation of ion-pairs with slightly enhanced ¹⁴N coupling. In contrast, tetra-n-butylammonium salts cause a gradual shift to smaller coupling constants. This is interpreted in terms of displacement of alcohol molecules by alkylammonium ions. In ethers, the ¹⁴N coupling constants for alkylammonium salts are far lower than those for alkali metal salts and there is a marked increase on going from caesium ions to lithium ions. However, the perturbation by alcohol solvent is much greater than that by lithium ions in ethereal solution. These results, together with results in mixed alcohol + ether solvents and various line-broadening phenomena, are discussed in terms of the nature of the ion-pairs involved.