Pressure-induced pseudorotation in crystalline pyrrolidine.

Pressure of 1.14(3) GPa leads to the crystallization of a new β phase of pyrrolidine, where every second molecule pseudorotates to the CH(2)-off envelope conformation, energetically less favored than the NH-off envelope. The β phase is triclinic, space group P ̅1, but it is isostructural with monoclinic phase α, where all molecules are in the N-off envelope conformation. The energy associated with the pseudorotation constitutes only about 5% of the total energy change. The energetically unfavorable axial N-H position is retained.