High-pressure polymorphism in pyridine.

Area of Science:

• Crystallography
• Materials Science
• Chemical Physics

Background:

• Pyridine exhibits complex high-pressure behavior.
• Previous studies identified high-pressure phases but lacked definitive structural data.

Purpose of the Study:

• To determine the crystal structures of pyridine phases II and III.
• To clarify the structural relationship and phase transitions between these high-pressure forms.
• To understand the molecular interactions and stability of these phases.

Main Methods:

• In situ single-crystal X-ray diffraction at high pressure (1.09 and 1.69 GPa).
• Neutron powder diffraction of deuterated pyridine (pyridine-d5) for equations of state.
• Raman spectroscopy for vibrational characterization.
• Rigid-body symmetry mode analysis and density functional theory (DFT) calculations.

Main Results:

• Crystal structure of phase III determined for the first time (space group P41212, Z'=½).
• Phase II structure confirmed (space group P212121, Z'=1).
• Phase III is topologically close-packed, explaining its stability over phase II.
• Raman spectra unambiguously assigned to each phase, resolving previous misidentifications.
• CH...π and CH...N interactions stabilize both phases, with preserved contacts across the III-II transition.

Conclusions:

• The crystal structures and phase transitions of high-pressure pyridine have been elucidated.
• Topological packing and intermolecular interactions govern the stability and behavior of pyridine under pressure.
• Accurate structural and vibrational data resolve previous ambiguities in high-pressure pyridine research.