1H, 13C and 17O NMR spectral study of chlorinated 3,4-dihydroxybenzaldehydes (protocatechualdehydes)

Abstract

Chlorinated 3,4-dihydroxybenzaldehydes have been studied by means of 1H, 13C and 17O NMR spectroscopy. The 1H and 13C NMR spectral assignments are based on 2-dimensional 13C- 1H chemical shift correlation spectroscopy (COSY). The 17O NMR measurements at natural isotope content in organic solvents are problematic owing to the poor solubility of the compounds and/or broadness of the resonance lines. In aqueous alkaline solutions, however, all protocatechualdehydes exhibit “easy-to-detect” 17O NMR spectral characteristics. The 17O NMR chemical shifts in the range of 140–480 p.p.m. are interpreted as arising from the different canonical structures of formyl substituted phenolate anions. The 13C NMR data reveal that the hydroxyl at the para-position to the formyl group is predominantly transformed to the corresponding phenolate anion in aqueous alkaline environment.