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 Δ_sol H _m, 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.