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

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 ΔpKa[= pKa(protonated base)– pKa(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 ΔpKa 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.