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

Infrared and Raman spectra of crystalline pyrrole and pyrrole-d5 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.