Vibrational study of crystal phase transitions in pyrrole (C₄H₅N) and C₄D₅N

Abstract

Infrared and Raman spectra of crystalline pyrrole and pyrrole-d5 were recorded in the range M O O 0 cm-'. The behaviour of internal and external modes was studied as a function of temperature between 200 and 5 K. Two phase transitions were observed at 67 and 30 K and their mechanism is discussed. For the incommensurate phase, between 67 and 30 K, the amplitude and phase modes were identified at about 20 and 10 cm-', respectively; for the first, a critical expownt of 030 was determined. No evidence of NH R hydrogen bond participation in the transitions has been detected.

Pressure-Induced Diversity of π-Stacking Motifs and Amorphous Polymerization in Pyrrole

“…At 0.6 GPa, the liquid sample crystallizes into solid crystal in the sample chamber of DAC. The refinement of the XRD pattern collected at 1.3 GPa has been carried out based on the reported low temperature structure with Pnma symmetry. , From Figure a, the herringbone N–H···π interactions strongly influence the crystal packing. The refined result identifies the symmetry of the solid phase as space group Pnma with the same structure at low temperature, which is shown in Figure b.…”

“…At 103 K, pyrrole crystallizes into phase N with space group Pnma. 14,15 In this paper, we present the first direct observations of pyrrole by means of in situ highpressure synchrotron XRD and Raman scattering spectra up to ∼34 GPa. A solid-to-solid transition is uncovered at about 6.2 GPa with a huge collapse of volume (∼40%) from Pnma to P2 1 /c, caused by the molecular rotational repacking of πstacking.…”