1H and 13C 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 1H chemical shift of the hydrogen‐bonded proton first increases with decreasing pKa 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 pKa of the phenol this proton becomes shielded again by the O atom of the TMAO molecule. This behaviour of the 1H 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 13C signals are considerably broadened, probably owing to the long lifetime of this bond. Δ14 Values were calculated from the 13C 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 13C and 1H NMR spectra. This difference is caused by the effects of the substituents on the 13C NMR signals, which are not taken into account in the calculation of the Δ14 values.