Vibrational study of crystal phase transitions in pyrrole (C 4 H 5 N) and C 4 D 5 N

Infrared and Raman spectra of crystalline pyrrole and pyrrole‐ d 5 were recorded in the range 5–4000 cm −1 . 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 −1 , respectively; for the first, a critical exponent of 0.30 was determined. No evidence of NH … π hydrogen bond participation in the transitions has been detected.