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Correlations between hydrogen bonds and solvent effects on phenol –OH proton shieldings, temperature coefficients (Δδ/ΔT) and effects on OH diffusion coefficients for numerous phenolic acids, flavonols, flavones, and oleuropein derivatives of biological interest were investigated in several organic solvents and were shown to serve as reliable indicators of hydrogen bonding and solvation state of –OH groups. The temperature coefficients span a range of −0.5 to −12.3 ppb K−1. Shielding differences of 2.0 to 2.9 ppm at 298 K were observed for solvent exposed OH groups between DMSO-d6 and CD3CN which should be compared with a shielding range of 7 ppm. This demonstrates that the solvation state of hydroxyl protons is a key factor in determining the value of the chemical shift. For –OH protons showing temperature gradients more positive than −2.5 ppb K−1, shielding changes between DMSO-d6 and CD3CN below 0.6 ppm, and diffusion coefficients significantly different from those of traces of H2O, there is an intramolecular hydrogen bond predictivity value of 100%. The C-3 OH protons of flavonols show very significant negative temperature coefficients and shielding changes between DMSO-d6 and CD3CN of 2.3 ppm, which indicate the absence of persistent intramolecular hydrogen bonds, contrary to numerous X-ray structures.
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Organic & Biomolecular Chemistry
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