Evidence of low temperature phase transition in 2,6-dimethylpyrazine - picric acid cocrystal by means of temperature dependent investigations: X-ray, DSC and IR.

Abstract In the present paper the structural and dynamical properties of crystalline 2,6–dimethylpyrazine - picric acid cocrystal are described. The title cocrystal crystallizes in centrosymmetric monoclinic P21/c space group. The X-ray diffraction and NMR spectroscopy show the presence of protonated form of 2,6-dimethypyrazine and deprotonated form of picric acid molecules. The Hirshfeld surface analysis for determine of intermolecular contacts was performed. Low temperature phase transition was observed and described by temperature dependent investigations: X-ray diffraction, different scanning calorimetry (DSC) and IR spectroscopy. The vibrational properties of cocrystal were investigated by use of IR and Raman spectroscopies, as well as density functional theory (DFT) with periodic boundary conditions. Activation energy for reorientational motion of CH3 group was determined from the band shape analysis of C CH3 bending vibration.