Molecular mechanism determining phase transitions in the 4-methyl-pyridine crystal

Abstract

Infrared and Raman spectra of the 4-methyl-pyridine crystal, and its deuterated derivative (d7), at various temperatures from 276.8K (melting point) to 5K are presented and vibrational assignments are proposed. Three crystal phases are observed, in the regions 5-100K (III), 100-254K (II) and 254-276.8K (I). Their structures and dynamics are discussed. In phase III, where the site symmetry for the molecule is C1, different potential-shapes are compared and only a nonperiodic function accounts satisfactorily for the five tau CH3and two tau CD3Raman bands. A molecular model is proposed where the methyl groups are correlated along infinite chains in the crystal. In phases II and I, where the molecular site symmetries are C2and C2v, respectively, the methyl group rotation is free. Quantitative models are developed to represent the temperature effect on frequencies or band widths observed below 150 cm-1and molecular mechanisms are proposed for the phase transitions. The ordering-disordering of the methyl groups which characterises the III to or from II transition is monitored by a rotational libration near 61 cm-1at 5K, strongly coupled to the tau CH3mode. The II to or from I transition is driven by a rotational libration near 116 cm-1.