Nature of hydrogen bonds formed by phenol derivatives and N,N-dimethylaniline in aprotic solvents : low-temperature NMR studies

Abstract

Phenol interact with N,N-dimethylaniline to produce complexes with molecular (O—H⋯N) and ion-pair (O^–⋯H—N⁺) hydrogen bonds. The degree of association and the character of the hydrogen bonds depends strongly on solution temperature and ΔpK_a[= pK_a(protonated base)– pK_a(phenol)] in water. The tautomeric equilibrium, O—H⋯N⇄O^–⋯H—N⁺, which is slow on the NMR timescale has been recorded in the 0.4 to –0.5 ΔpK_a range below 140 K. Thermodynamic and kinetic quantities of the equilibrium were calculated from ¹H NMR spectra of the 2,3,5,6,-tetrachlorophenol complex. The thermodynamic quantities for the 2,6-dichloro-4-nitrophenol complex were obtained from ¹³C NMR results.