A curious case of dynamic disorder in pyrrolidine rings elucidated by NMR crystallography

A pharmaceutical exhibits differing dynamics in crystallographically distinct pyrrolidine rings despite being nearly related by symmetry, with one performing ring inversions while the other is constrained to torsional librations. Using C-13 solid-state magic-angle spinning (MAS) NMR and DFT calculations, we show that this contrast originates from C-HMIDLINE HORIZONTAL ELLIPSISH-C close contacts and less efficient C-HMIDLINE HORIZONTAL ELLIPSIS pi intermolecular interactions observed in the transition state of the constrained pyrrolidine ring, highlighting the influence of the crystallographic environment on the molecular motion.