The Polymorphism of Glycine. Thermochemical and structural aspects

Abstract

X-ray, DSC and solution calorimetric investigations were carried out for α-, β- and γ-modifications of glycine. Particular attention was paid to kinetic and thermochemical aspects of γ- → α-phase transition. The temperature of this phase transition turned out to be sensitive to a) conditions under which the crystals of the γ-modification were grown, b) tempering of crystals c) form (geometry) of crystals. Kinetics of this phase transition of single crystals of γ-phase in rhomboedric form can be described by the equation for two-dimension nuclei growth, whereas for crystals of triangle geometry the equation for three dimension growth is valid. On the basis of energy parameters describing growth of α-form in γ- →α-phase transition, the kind of structure defects, which are responsible for this phase transition, was estimated. Taking into account the ΔsolHm, the absolute values of the lattice energies of the investigated polymorphs indescending order are follows: γ->α->β-modification. The obtained results are discussed with respect to the peculiarity of the crystal lattice structures, particularly the network of hydrogen bonds. The β-modification of glycine is monotropically related to the other forms, whereas γ-and α-polymorphs are enantiotropically-related phases.