Intramolecular Hydrogen Bonds of the CO…︁H?O Type as Studied by 17O-NMR

The ¹⁷O-NMR spectra of 1,4-naphthoquinone and 5-hydroxy-1,4-naphthoquinone (juglone) have been recorded in CDCl₃ solution at 40°. In juglone the ¹⁷O resonance of the carbonyl peri to the OH group was displaced by 70 ppm to low frequency relative to the resonance in the para-position. It is shown that this chemical shift arises mainly from intramolecular H-bonding, the substituent and steric effects being one order of magnitude smaller. Large carbonyl ¹⁷O chemical shifts between ?34 and ?100 ppm were also observed in a series of aromatic aldehydes and ketones where intramolecular H-bonds of the C?O…?H? O type are formed. The H-bond-induced carbonyl ¹⁷O chemical shifts were linearly correlated with both the ¹⁷O and ¹H chemical shifts of the OH groups. They represent a most sensitive measure of the strength of intramolecular H-bonds. The ¹⁷O resonances of the OH groups were directed to high frequency on H-bonding. Analysis of the ¹⁷O chemical shifts in 2,2′-dihydroxy-benzophenone showed clearly that the two OH groups build H-bonds simultaneously to the single carbonyl group. The ¹⁷O linewidths decreased strongly on H-bonding; the linewidth of the H-bonded carbonyl O-atom in juglone, for example, was reduced by 25% with respect to that of the free carbonyl O-atom. The carbonyl O-atom quadrupole coupling constants in juglone, evaluated from the combined use of ¹³C and ¹⁷O relaxation times, were 9.5 and 11.0 MHz, respectively. No correlation was observed between the H-bond-induced ¹⁷O chemical shifts and the variations in ¹⁷O quadrupole coupling constants.