On the structure and spectroscopic properties of sparteine and its diastereoisomers

The equilibrium structures of the lupine alkaloid sparteine, its diastereoisomers (a-sparteine and b-sparteine), and two related tricyclic diamines were investigated with the density functional theory ansatz B3LYP and HF-MP2 method. With the exception of b-sparteine, these free molecules are structurally simple, since one conformer predominates both in the gas phase and in solution. The mono-protonated forms of all these molecules are characterized by intramolecular asymmetric hydrogen bonds. The di-protonated species have the same structural preferences as the parent free bases. All the theoretical structural predictions are consistent with the available experimental results. The NMR chemical shifts, calculated by the continuous set of gauge transformations formalism with the DFT/B3LYP method, are fairly consistent with NMR observation. The He(I) photoelectron spectra were measured and assigned by means of ab initio outer valence Greens function calculations. The overwhelming predominance of the lowest-energy conformer of all compounds is confirmed by various spectroscopic results. However, the structure of b-sparteine is very special. The 13C NMR spectra in solution at room temperature are indicative of an effective C2 molecular symmetry. From the analysis of the NMR spectra from +20 to )85 C it is inferred that the most stable C1 conformer of b-sparteine is almost exclusively present in solution. At room temperature, it rapidly interconverts between two enantiomeric forms to produce the NMR spectrum of a time-averaged C2 conformer.