Theoretical and experimental NMR study of protopine hydrochloride isomers

The H-1 and C-13 NMR chemical shifts of cis- and trans-protopinium salts were measured and calculated. The calculations of the chemical shifts consisted of conformational analysis, geometry optimization (RHF/6-31G** method) and shielding constants calculations (B3LYP/6-31G** method). Based on the results of the quantum chemical calculations, two sets of experimental chemical shifts were assigned to the particular isomers. According to the experimental results, the trans-isomer is more stable and its population is similar to 68%. Copyright (c) 2005 John Wiley & Sons, Ltd.