Intramolecular Hydrogen Bonds of the CO…HO 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.