Variable‐temperature NMR studies of 2,6‐dihydroxy acylaromatic compounds. Deuterium isotope effects on chemical shifts, isotopic perturbation of equilibrium and barriers to rotation

Abstract

AbstractA series of 2,6‐dihydroxy acylaromatic compounds were investigated to characterize the rotational and hydrogen bonding properties of the carbonyl group. Deuterium isotope effects on 1H and 13C chemical shifts due to deuteriation of OH groups were determined at both ambient and low temperature. In the latter case isotope effects on chemical shifts of the individual rotamers can be determined. Deuteriation of one of the OH groups may lead to isotopic perturbation of the tautomeric equilibrium of the carbonyl group and the two hydroxyl groups. The perturbation was found to be larger in ketones than in esters. Complete band shape analysis of the OH resonances of the esters and ketones in a temperature interval above and below the coalescence temperature led to ΔG≠, AH≠ and ΔS≠ values for various concentrations of added THF‐d8. ΔS≠ was found to be strongly negative. Temperature coefficients for the shift of the OH resonances showed large variations for esters and ketones owing to the different hydrogen bond patterns. The esters have two intramolecular hydrogen bonds, one strong and an additional weaker one between the OH and OR groups. The second OH group of the ketones was shown to point primarily towards C‐5. Increasing amounts of THF‐d8 increased the amount of this rotamer. The anisotropy of the XCO group at C‐2, C‐6 was shown to lead to a low‐field shift of C‐2, very different from that found for CO groups without hydrogen bonds. The anisotropy caused by OH groups can also be estimated. On the basis of the thermodynamic parameters, a model for the rotation of the ester group is suggested. The rate‐determining step involves both intramolcular hydrogen bonds, which are twisted out of the ring plane to form hydrogen bonds to the solvent or other hydrogen bond acceptors.