Abstract: In this study, NMR techniques, including ¹H NMR, ¹³C NMR, DEPT, ¹⁹F NMR, HSQC, and HMBC, were used in combination to assign the ¹H and ¹³C chemical shifts of lansoprazole in room temperature. On the basis of the assignments and the results of variable-temperature experiments, a hypothesis was put forward to explain the existence of several low broad lines in the ¹H and ¹³C NMR spectra of the compound. It was proposed that there exist two inter-converting tautomers at room temperature. The inter-conversion rate increased and the broad lines became narrower as the temperature increased, and vice versa. The low broad lines in the ¹H spectra of the compound disappeared at -70 ℃, indicating the existence of only one tautomer and the termination of the inter-conversion process. The ¹H spectra of lansoprazole obtained at -70 ℃ were assigned, which were shown to be consistent with the results obtained at in room temperature.

Key words: NMR, assignments, 2D NMR, lansoprazole, tautomerism