Solid-State Phase Transitions of dl-Aminobutyric Acid

• Published in: The journal of physical chemistry. B 2012-09, Vol.116 (35), p.10715-10721
• Main Authors: Görbitz, Carl Henrik, Alebachew, Fassil, Petříček, Václav
• Format: Article
• Language: English
• Subjects: Amino acids; Aminobutyrates - chemistry; Calorimetry, Differential Scanning; Chains; Conversion; Crystal structure; Crystallography; Crystallography, X-Ray; Disorders; Hydrogen Bonding; Phase transformations; Phase Transition; Physical chemistry; Stereoisomerism; Thermodynamics; Transition Temperature
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp305016k

Four polymorphs are known for the amino acid racemate dl-aminobutyric acid: one tetragonal form B and three monoclinic forms A, C, and D for which two solid–solid phase transitions take place between 209 and 337 K. The intermediate form A, in space group P21/c, is known to display disorder in the crystalline state with the ethyl side chain occupying three different conformations. The high temperature form D and the low temperature form C have been assumed to display a similar type of disorder in the space group C2/c. The present investigation, on the basis of high quality crystal structures at 100 K (C form), 220 K (A form), 295 K (A form), and 295 K (D form), provides updated results for all three forms and in particular demonstrates that the A → C and A → D transitions in fact involve only a partial space group conversion, so that two monoclinic phases are simultaneously present at high and low temperatures. To the best of our knowledge, such a phenomenon has not been observed previously for an organic molecule. The nonroutine refinement of the crystallographic data was performed with a specially adapted version of the program JANA2006. The observed polymorphs are compared with those found for dl-norvaline, dl-norleucine, and dl-methionine.