Pressure-induced pseudorotation in crystalline pyrrolidine.

Area of Science:

• Solid-state chemistry
• Crystallography
• Molecular conformation

Background:

• Pyrrolidine is a cyclic amine with known conformational flexibility.
• Understanding molecular behavior under high pressure is crucial for materials science.

Purpose of the Study:

• To investigate the structural and conformational changes of pyrrolidine under high-pressure conditions.
• To characterize the new crystalline phase formed at 1.14 GPa.

Main Methods:

• Single-crystal X-ray diffraction under high pressure.
• Computational analysis of molecular conformations and energies.

Main Results:

• A new triclinic β phase of pyrrolidine was discovered at 1.14(3) GPa.
• In the β phase, every second molecule adopts a CH(2)-off envelope conformation, which is energetically less favorable than the NH-off envelope found in the α phase.
• The β phase is isostructural with the monoclinic α phase, space group P ̅1.
• The energy contribution from pseudorotation is minimal (approx. 5%) compared to the total energy change.
• The energetically unfavorable axial N-H position is maintained.

Conclusions:

• High pressure forces pyrrolidine into a less stable conformation, highlighting pressure-induced molecular distortions.
• The structural similarity between the α and β phases suggests a subtle but significant conformational shift under pressure.
• The retention of the axial N-H position indicates strong directional bonding or packing forces even at high pressure.