Premium
1 H and 13 C NMR studies of the proton transfer in complexes of substituted phenols with trimethylamine N ‐oxide
Author(s) -
Brycki Bogumil,
Brzezinski Bogumil,
Zundel Georg,
Keil Thomas
Publication year - 1992
Publication title -
magnetic resonance in chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.483
H-Index - 72
eISSN - 1097-458X
pISSN - 0749-1581
DOI - 10.1002/mrc.1260300609
Subject(s) - chemistry , proton nmr , proton , hydrogen bond , phenol , phenols , chemical shift , trimethylamine , carbon 13 nmr , hydrogen atom , nmr spectra database , molecule , oxide , photochemistry , spectral line , stereochemistry , organic chemistry , physics , alkyl , quantum mechanics , astronomy
Substituted phenol–trimethylamine N ‐oxide (TMAO) systems were studied. The 1 H chemical shift of the hydrogen‐bonded proton first increases with decreasing p K 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 p K a of the phenol this proton becomes shielded again by the O atom of the TMAO molecule. This behaviour of the 1 H 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 13 C signals are considerably broadened, probably owing to the long lifetime of this bond. Δ 14 Values were calculated from the 13 C 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 13 C and 1 H NMR spectra. This difference is caused by the effects of the substituents on the 13 C NMR signals, which are not taken into account in the calculation of the Δ 14 values.