LONG-RANGE DEUTERIUM-ISOTOPE EFFECTS ON C-13 CHEMICAL-SHIFTS OF INTRAMOLECULARLY HYDROGEN-BONDED COMPOUNDS - PURPUROGALLINS

Long-range deuterium isotope effects on C-13 chemical shifts, (n)DELTAC(OD), were studied in the intramolecularly hydrogen-bonded purpurogallins (benzotropolones). A very large long-range isotope effect from the hydrogen-bonded 4-OH(D) is observed over six bonds at C-7. Further, long-range isotope effects are transmitted over as many as eight bonds. The transmission pathway including the carbonyl carbon is favoured, and some of the long-range effects are transmitted via the carbonyl group. Large long-range isotope effects are thus observed in hydrogen-bonded systems with effective conjugation to the carbonyl group via alternating double bonds. This study indirectly supports recent theoretical predictions. The C-13 and H-1 NMR spectra of purpurogallin and its 2,3-dimethyl, 2,3,6-trimethyl and 2,3,4,6-tetramethyl derivatives are assigned, and the hydrogen bond patterns are established based on the OH chemical shifts and 2DELTAC(OD) isotope effects.