Rotational spectra of tetracyclic quinolizidine alkaloids: does a water molecule flip sparteine?Electronic supplementary information (ESI) available: Fig. S1-S3, computational data (Tables S1-S3), experimental transitions (Tables S4-S6), rotational parameters for the 18O water isotopologue (Table S7), atomic coordinates (Tables S8-S15) and binding energies (Table S16). See DOI: 10.1039/c7cp01432e

Sparteine is a quinolizidine alkaloid used as a chiral auxiliary in asymmetric synthesis. We examine whether hydration by a single molecule can flip sparteine from the most stable trans conformation to the bidentate cis arrangement observed in catalytic complexation to a metal center. Sparteine and the sparteine-water dimer were generated in a supersonic jet expansion with H 2 16 O and H 2 18 O, and characterized by broadband chirped-pulse microwave spectroscopy. Even though the bidentate water dimer was predicted with larger binding energy, a single isomer was observed for the monohydrated cluster, with sparteine retaining the trans conformation observed for the free molecule. The absence of the bidentate dimer is attributed to the kinetic control of cluster formation, favoring the pre-expansion most abundant monomer. The structural properties of the O-H N hydrogen bond in the dimer are compared with those of complexes of other secondary and tertiary amines. Flipping or not flipping? The sparteine-water dimer generated in a jet expansion retains the trans conformation of the parent molecule.