Dynamics and thermodynamics of crystalline polymorphs. 3. γ-Glycine, analysis of variable-temperature atomic displacement parameters, and comparison of polymorph stabilities.

In a series of systematic studies, we have investigated the molecular motion in crystals of the glycine polymorphs and determined their thermodynamic functions from an analysis of multitemperature atomic displacement parameters (ADPs) combined with ONIOM calculation on 15-molecule clusters. The studies are aimed at providing insight into the factors governing the relative stabilities of the α-, β-, and γ-polymorphs. This Article, the last in the series, focuses on the most stable polymorph, γ-glycine. Multitemperature diffraction data of the γ-glycine polymorph have been collected to 0.5 A resolution between 10 and 300 K at two synchrotron beamlines, KEK Photon Factory and ID11 of the ESRF. The ADPs of γ-glycine from these sources differ significantly, as previously observed also for the other two polymorphs. A simple model of rigid body motion explains the ADPs from KEK and their temperature dependence. It provides lattice vibration frequencies that are in line with those from Raman spectroscopy. Togethe...