Molecular packing modes. Part VI. Crystal and molecular structures of two modifications of tetrolic acid

The crystal structures of the α- and β-forms of tetrolic acid have been determined from X-ray diffractometer data. Crystals of the α-form are triclinic, space group P, Z= 2, a= 7·320, b= 5·099, c= 7·226 Å, α= 83·97, β= 117·46, γ= 112·00°. The structure was refined to R 0·07 for 985 reflections. The molecules form hydrogen-bonded cyclic dimers (O ⋯ O 2·649 Å) in which the C–O bond lengths are almost equal (1·252, 1·265 Å). ‘Half hydrogens’ were attached to each oxygen, their parameters refined, and their presence substantiated by a difference-Fourier synthesis. The methyl hydrogen atoms were located.The structure of the monoclinic β-modification [a= 7·887, b= 7·121, c= 3·937 Å, β= 100·18°, Z= 2, P21] was refined from counter data to R 0·04 (687 reflections). The methyl hydrogen atoms were located. The bond lengths within the carboxylic group are distinct: CO 1·204, C–O 1·310 Å; the hydroxy-hydrogen was clearly evident. The molecules form a continuous array of O–H ⋯ O hydrogen bonds (O ⋯ O 2·655 Å) between molecules related by a two-fold screw axis similar to the arrangement in the structure of formic and acetic acids.The carbon–carbon bond lengths of the α- and β-forms are: H3C–C 1·458, 1·455, CC 1·182, 1·178, and C–CO2H 1·441, 1·441 Å.