<SUP>17</SUP>O NMR spectroscopy: study of intramolecular hydrogen bonding in phenols and salicylaldehydes

Abstract

Natural abundance <SUP>17</SUP>O NMR data for fifteen 2- and 4-substituted phenols, ten 3-and 5-substituted 2-hydroxybenzaldehydes and eight 3-substituted benzaldehydes, recorded at 75&#176;C in acetonitrile are reported. The chemical shift change due to intramolecular hydrogen bonding for the phenolic oxygen was found to be 10-14 ppm shielding. In acetonitrile, the <SUP>17</SUP>O NMR chemical shift for phenol signals was insensitive to added water up to water concentrations of 0.5 mole fraction. The <SUP>17</SUP>O NMR chemical shifts of the 4-substituted phenols gave an excellent correlation (r = 0.990) with anisole <SUP>17</SUP>O NMR data; the data also correlated moderately well with &#963;<SUP>-</SUP> (r = 0.974). The chemical shifts of the 3-substituted benzaldehydes were correlated with &#963;<SUP>+</SUP> values (r = 0.991). A plot of the carbonyl chemical shift data for the substituted 2-hydroxybenzaldehydes versus the carbonyl data for 3-substituted benzaldehydes gave a slope of 0.87 and with r = 0.960. The plot of the 4-substituted phenol data with that for OH of the corresponding 2-hydroxybenzaldehydes gave a slope of 1.04 with r = 0.996. Proton to oxygen coupling for the phenolic group of several of the intramolecular hydrogen bonded systems was observed directly [J(OH) = 58-92 Hz]. MM2 and MOPAC calculations predict that the hydrogen bond distances and angles for the substituted 2-hydroxybenzaldehydes and the partial atomic charges for the carbonyl groups (AMI) were essentially constant. After corrections for electronic effects the chemical shift changes due to hydrogen bonding for the donor (&#916;&#948;<SUB>HBD</SUB>) and acceptor (&#916;&#948;<SUB>HBA</SUB>) of the carbonyl-phenol intramolecular bonding system were 5-12 and 30 &#177; 2 ppm, respectively. The &#916;&#948;<SUB>HBA</SUB> value was between those for keto and ester acceptors consistent with the relative basicity of the aldehyde group. The &#916;&#948;<SUB>HBD</SUB> value was substantially larger than those for phenolic donors to keto and ester groups