Crystal Structure Study of the Metastable β-modification of Glycine and its Transformation into the α-modification

Abstract

Crystal structure refinement was performed of the metastable β‐modification of glycine (space gr. P2₁, a = 5.092(2) Å, b = 6.273(3) Å, c = 5.384(3)Å, β = 113.17(4)°, Z = 2, R = 0.0274). The crystals were obtained by a new method (not described in the literature): they were precipitated from a water solution with the addition of glacial acetic acid. The crystal structure of the glycine α‐modification [space gr. P2₁/n, a =5.106(1) Å, b = 11.979(5) Å, c = 5.463(2) Å, β = 111.75(2)°:, Z =4] was refined for comparison. Transition from the β‐ to the α‐modification in a damp atmosphere was revealed, resulting in the formation of a strongly strained crystal of a mosaic structure. The orientation of the crystallographic axes in the newly formed crystal of the α‐modification relative to the axes in the initial crystal of the β‐modification was determined. It is shown that as in α‐glycine, the lengths of the intramolecular C—O bonds in the β‐modification are fairly similar [1.248(2) and 1.253(2) Å] and the previously found significant difference between them is most likely due to the errors of the photomethod. A comparative analysis of the crystal structures of the α‐ and β‐glycine modifications (glycine zwitterion packings and structure of intermolecular hydrogen bond networks) was performed. Data of the analysis are used to discuss the properties of the glycine modifications, the possible reasons for the stability of the β‐modification in dry air, the difficulties of crystal growth from solutions, and the ease of crystallization of the α‐modification under various conditions.