Structural distortion of the α, β, and γ polymorphs of glycine on cooling

The effect of cooling down to 150 K on the structures of alpha- (s.g. P2(1)/n), beta- (s.g. P2(1)), and gamma- (s.g. P3(1)) polymorphs of glycine was studied by X-ray single-crystal diffraction. No polymorphic transformations were detected. Relative volume changes and the anisotropy of structural distortion of the three polymorphs of glycine were compared. Maximum contraction of structures was measured in the b-direction for the alpha- and beta-polymorphs, and in the a-direction for the gamma-polymorph. Minimum contraction in the gamma-polymorph was measured in c-direction. The structures of the alpha- and the beta-polymorphs slightly expanded on cooling in the directions close to c. The directions of maximum and minimum lattice strain were related to the directions of weak and strong hydrogen bonds in the structures. Crystal structures for the three polymorphs were refined at 294 K and at 150 K. The effects of cooling on the intramolecular geometry of glycine molecules and on the intermolecular hydrogen bonds in all the three polymorphs were compared. Structural distortion on cooling was compared with that on increasing hydrostatic pressure.