H-1, C-13 AND O-17 NMR SPECTRAL STUDY OF CHLORINATED 3,4-DIHYDROXYBENZALDEHYDES (PROTOCATECHUALDEHYDES)

Chlorinated 3,4-dihydroxybenzaldehydes have been studied by means of H-1, C-13 and O-17 NMR spectroscopy. The 1H and C-13 NMR spectral assignments are based on 2-dimensional C-13-H-1 chemical shift correlation spectroscopy (COSY). The O-17 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, ah protocatechualdehydes exhibit ''easy-to-detect'' O-17 NMR spectral characteristics. The O-17 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 C-13 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.