5-Butyl-5-ethylbarbituric acid: A phase transition at low temperature

The room-temperature crystal structure of 5-butyl-5-ethyl-barbituric acid (generally known as butobarbitone), C₁₀H₁₆-N ₂O₃, was reported in space group C2/c [Bideau (1971). C. R. Acad. Sci. Paris Ser. C, 272, 757-760]. A redetermination at 120 K using synchrotron radiation shows the space group at this temperature to be P2 ₁/n and not C₂/c. There are two crystallographically independent molecules in the asymmetric unit, but no solvent. Reported issues concerning possible disorder of the molecule are addressed; the butyl substituent of one of the molecules adopts an unusual conformation in being not fully extended. A subsequent re-collection at room temperature shows that the space group is indeed C2/c (A2/a with the axes selected in this report), and so the crystal structure undergoes a phase change upon cooling to 120 K.