Unprecedented Ultra-High-Resolution Hydroxy Group ¹H NMR Spectroscopic Analysis of Plant Extracts

A general method is demonstrated for obtaining ultra-high resolution in the phenolic hydroxy group ¹H NMR spectroscopic region, in DMSO-<i>d</i>₆ solution, with the addition of picric acid. Line-width reduction by a factor of over 100 was observed, which resulted in line-widths ranging from 1.6 to 0.6 Hz. This unprecedented resolution, in combination with the shielding sensitivity of the hydroxy group absorptions to substituent effects at least up to 11 bonds distant and the application of 2D ¹H–¹³C HMBC techniques, allows the unequivocal structure analysis of natural products with phenolic hydroxy groups in complex plant extracts