PROTON CHEMICAL-SHIFT ASSIGNMENTS IN CITRATE AND TRIMETHYL CITRATE IN CHIRAL MEDIA

The citrate ion gives rise to four different methylene proton chemical shifts in the presence of (S)-lactate and Pr3+. Trimethyl citrate behaves similarly in the presence of (S)-2,2,2-trifluoro-1-(9-anthryl)ethanol. The four methylene shifts have been assigned in an absolute way by comparisons with those from the corresponding spectra of (2R,3R)-citrate-2-d and trimethyl (2R,3R)-citrate-2-d, respectively. The chemical shifts of the three methyl groups in trimethyl citrate in the presence of the anthryl shift reagent have also been assigned. Deuterium isotope effects on the proton chemical shifts of these molecules have been determined. In the absence of shift reagents these effects are mainly of the intrinsic type, but in their presence there are equilibrium perturbation contributions. The H-1 NMR line widths and chemical shifts in the citrate-lactate-PrCl3 system depend strongly on the pH, spectrometer frequency, and temperature, with the best results obtained at room temperature, pH almost-equal-to 3.8, and a spectrometer frequency of 200 MHz.