Conformational Equilibrium of Cinchonidine in C₆D₁₂ Solution. Alternative NMR/DFT Approach

¹H NMR and ¹³C NMR chemical shifts as well as conformation dependent vicinal ¹H–¹H spin–spin coupling constants for cinchonidine in a dilute C₆D₁₂ solution have been measured. These data have been interpreted in detail exploiting the results of the extensive quantum chemistry calculations of molecular geometry and NMR parameters of the molecule, performed using the density functional theory (DFT) B3LYP/6-311++G(2d,p) polarizable continuum model (PCM) level of theory. The experimental values of NMR parameters for cinchonidine have been reproduced very well in terms of parameters calculated for key conformers of this molecule. Simultaneously, the analysis has provided us with a lot of information on conformational equilibrium of cinchonidine in the investigated solution. These findings remain in general agreement with the conclusions of other works, based on NOESY spectra or other physicochemical data. Thus, a careful quantitative interpretation of easily measurable NMR chemical shifts can be an independent and valuable source of structural information even in such complex cases as cinchonidine in solution.