Unexpected Fractionation in Site-Specific ¹³C Isotopic Distribution Detected by Quantitative ¹³C NMR at Natural Abundance

The recent development of 13 C NMR to measure absolute 13 C/ 12 C ratios at natural abundance has made it possible to examine site-by-site variation at natural abundance. This technique has been applied to two common physical processes: distillation and column chromatography. It is shown that the target molecules, ethanol and vanillin, respectively, both show isotopic fractionation that differs in size and/or in sense at each carbon atom. The observation of this disparity in behavior is novel and unexpected. 13 C SNIF-NMR appears as a new effective tool to study noncovalent isotope effects (NCIE) on target molecules.