title: Solid-state phase transitions of DL-aminobutyric acid.
abstract: 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 P2(1)/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.
authors.affiliation: Department of Chemistry, University of Oslo, N-0315 Oslo, Norway. c.h.gorbitz@kjemi.uio.no
authors.forename: Carl Henrik
authors.initials: CH
authors.lastname: Görbitz
authors.forename: Fassil
authors.initials: F
authors.lastname: Alebachew
authors.forename: Václav
authors.initials: V
authors.lastname: Petříček
mesh_terms: D000613:Aminobutyrates / Q000737:chemistry*; D002152:Calorimetry, Differential Scanning; D018360:Crystallography, X-Ray; D006860:Hydrogen Bonding; D044367:Phase Transition; D013237:Stereoisomerism; D013816:Thermodynamics; D044366:Transition Temperature
chemical_list: D000613:Aminobutyrates