H-1 AND C-13 NMR-STUDIES OF PROTON-TRANSFER IN INTRAMOLECULAR HYDROGEN-BONDS IN SUBSTITUTED 2-DIETHYLAMINOMETHYLPHENOL N-OXIDES

Eleven substituted 2-diethylaminomethylphenol N-oxides were studied in CDCl3 solutions by H-1 and C-13 NMR spectroscopy. The H-1 chemical shifts of the intramolecular hydrogen-bonded proton and the DELTA14 values obtained from the C-13 signals were considered as a function of the pK(a) values of the parent phenols. The H-1 chemical shift of the hydrogen-bonded proton shows a maximum at pK(a) almost-equal-to 7. This result agrees well with the turning point of the curve if DELTA14 is plotted as a function of the pK(a) of the parent phenols. All these results agree well with recently obtained Fourier transform IR results. With the system with the strongest hydrogen bond (pK(a) almost-equal-to 7) a broad, flat, single-minimum proton potential or a double-minimum proton potential with a small barrier is present. With this system the deshielding of the proton in the intramolecular hydrogen bond is strongest.