H-1-NMR AND C-13 NMR-STUDIES OF THE PROTON-TRANSFER IN COMPLEXES OF SUBSTITUTED PHENOLS WITH TRIMETHYLAMINE N-OXIDE

Substituted phenol-trimethylamine N-oxide (TMAO) systems were studied. The H-1 chemical shift of the hydrogen-bonded proton first increases with decreasing pK(a) of the phenols and, after a sharp maximum, it then decreases. Thus, the shielding of the hydrogen-bonded proton by the O atom of the phenol first decreases, and this proton is most deshielded in the 3,4-dinitronphenol-TMAO system. With a further decrease in the pK(a) of the phenol this proton becomes shielded again by the O atom of the TMAO molecule. This behaviour of the H-1 NMR signal is in very good agreement with the changes in the shape of the proton potential obtained from IR spectra. In the case of the strongest hydrogen bond, in the 3,4-dinitrophenol-TMAO system, the C-13 signals are considerably broadened, probably owing to the long lifetime of this bond. DELTA-14 Values were calculated from the C-13 NMR signals, which should also reflect the changes in the nature of the phenol-TMAO hydrogen bonds. A small difference is obtained between the results obtained from the C-13 and H-1 NMR spectra. This difference is caused by the effects of the substituents on the C-13 NMR signals, which are not taken into account in the calculation of the DELTA-14 values.