Abstract
Experimental parameters that influence the resolution of 1H-NMR phenol OH signals are critically evaluated with emphasis on the effects of pH, temperature and nature of the solvents. Extremely sharp peaks (Δν1/2 ≤ 2 Hz) can be obtained under optimized experimental conditions which allow the application of 1H-13C HMBC-NMR experiments to reveal long range coupling constants of hydroxyl protons and, thus, to provide unequivocal assignment of the OH signals even in cases of complex polyphenol natural products. Intramolecular and intermolecular hydrogen bonds have a very significant effect on 1H OH chemical shifts which cover a region from 4.5 up to 19 ppm. Solvent effects on –OH proton chemical shifts, temperature coefficients (Δδ/ΔT), OH diffusion coefficients, and nJ(13C, O1H) coupling constants are evaluated as indicators of hydrogen bonding and solvation state of phenol –OH groups. Accurate 1H chemical shifts of the OH groups can be calculated using a combination of DFT and discrete solute-solvent hydrogen bond interaction at relatively inexpensive levels of theory, namely, DFT/B3LYP/6-311++G (2d,p). Excellent correlations between experimental 1H chemical shifts and those calculated at the ab initio level can provide a method of primary interest in order to obtain structural and conformational description of solute-solvent interactions at a molecular level. The use of the high resolution phenol hydroxyl group 1H-NMR spectral region provides a general method for the analysis of complex plant extracts without the need for the isolation of the individual components.
Highlights
Hydrogen bonding is a fundamental aspect of chemical structure, conformation and reactivity [1,2,3,4].Detection of hydrogen bonds, remains an area of active research
Proton exchange rates in alcohol –OH groups can be reduced by dissolving in DMSO-d6 or acetone-d6 [7], by supercooling aqueous solutions [8] or by using organic co-solvents [9] and, have already been utilized in structural analysis of carbohydrates [9,10]
Shapetko and Shigorin [11] provided a brief account of 1H-NMR studies of intramolecular hydrogen bonds of derivatives of hydroxynaphthoquinones, hydroxyanthraquinones, and tropolone which are an important class of natural products
Summary
Hydrogen bonding is a fundamental aspect of chemical structure, conformation and reactivity [1,2,3,4]. The use of hydroxyl protons in hydrogen bonding and conformational NMR studies in solution, presents experimental challenges due to rapid chemical exchange between hydroxyl groups and protic solvents. Shapetko and Shigorin [11] provided a brief account of 1H-NMR studies of intramolecular hydrogen bonds of derivatives of hydroxynaphthoquinones, hydroxyanthraquinones, and tropolone which are an important class of natural products. In the present review article we will summarize 1H-NMR experimental parameters that influence the resolution of phenol –OH groups and provide an overview of recent developments in the use of. H-NMR as structural, conformational and analytical tool of intra- and inter-molecular hydrogen bonds of phenol containing natural products and model compounds
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