Effect of protonation on the conformation of cinchonidine.

The protonation of cinchona alkaloids in solution leads to the severe restriction of their internal rotational degrees of freedom and to the locking of the molecule around a specific conformation held in place by a bridging counterion of the acid used for protonation. For HF, direct interactions were detected by NMR between the fluorine anion and not only with the acidic hydroxo group but also with non-acidic hydrogen atoms in the quinoline ring.