¹⁷O NMR spectroscopy: study of intramolecular hydrogen bonding in phenols and salicylaldehydes

Abstract

Natural abundance ¹⁷O NMR data for fifteen 2- and 4-substituted phenols, ten 3-and 5-substituted 2-hydroxybenzaldehydes and eight 3-substituted benzaldehydes, recorded at 75°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 ¹⁷O NMR chemical shift for phenol signals was insensitive to added water up to water concentrations of 0.5 mole fraction. The ¹⁷O NMR chemical shifts of the 4-substituted phenols gave an excellent correlation (r = 0.990) with anisole ¹⁷O NMR data; the data also correlated moderately well with σ^- (r = 0.974). The chemical shifts of the 3-substituted benzaldehydes were correlated with σ⁺ 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 (Δδ_HBD) and acceptor (Δδ_HBA) of the carbonyl-phenol intramolecular bonding system were 5-12 and 30 ± 2 ppm, respectively. The Δδ_HBA value was between those for keto and ester acceptors consistent with the relative basicity of the aldehyde group. The Δδ_HBD value was substantially larger than those for phenolic donors to keto and ester groups.