The re-entrant phase transitions in crystalline malononitrile, CH2(CN)2: a neutron powder diffraction study
The temperature dependence of the crystal structures of the low-temperature α-phase and the intermediate-temperature β-phase of crystalline malononitrile, CH2(CN)2, have been determined using Rietveld refinement on neutron powder diffraction data. The results confirm that the α-phase has the same structure as the high-temperature γ-phase, and the refined structure of the β-phase is in agreement with previous structure determination using single-crystal x-ray diffraction. The variations of the structure with temperature due to the phase transitions are identified and analysed in terms of molecular rotations and spontaneous strains.
Vibrational study of phase transition of solid malononitrile
Solid phases and phase transitions of crystals of malononitrile, CH2(CN)2, have been studied by Raman and infrared spectroscopy. Among twelve Raman active lattice modes of T′ phase a soft mode has been identified and assigned to a R′b libration. The T′–F–T transitions can be considered as second order and of displacive type since the soft mode frequency decreases continuously to zero at Tc when the transition temperature is approached from either above or below. Critical exponent β∼1/2 has been determined from the soft mode frequency versus Tc‐T plot. Half‐width of the soft mode band has also been studied as a function of temperature and relaxation times have been extracted from the corresponding curves. The stable low temperature phase L has been characterized spectroscopically: unlike T′, F, T phases it contains CH...N hydrogen bonds. The behavior of its lattice frequencies with temperature indicates that L–F phase change is a first order transition.
Spectroscopic Intensity and Dipole Moment Study on Malononitriles
The Infrared and Raman intensities of the 2250 cm−1 band in malononitrile and some of its derivatives has been determined. 
 
 
 
The dipole moments of these compounds have been measured in benzene solutions. The results are compared mutually and are discussed in terms of the inductive and electron delocalisation effects of the substituents.