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 CH₃ and two tau CD₃ Raman 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 C₂ and C_2v, 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^-1 and 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^-1 at 5K, strongly coupled to the tau CH₃ mode. The II to or from I transition is driven by a rotational libration near 116 cm^-1.