The crystal structures of trimellitic anhydride and two of its solvates

The crystal structure of trimellitic anhydride (TMA) has been solved using Monte Carlo simulated annealing techniques, and refined using synchrotron p owder diffraction data. The crystal structure of TMA consists of helical hy drogen-bonded chains parallel to b. Prominent in the structure are C-H ... O=C hydrogen bonds between aromatic hydrogens and the carbonyl groups of th e anhydride. The structures of two solvates of TMA have been determined usi ng single crystal techniques. TMA:dimethylformamide (TMA: DMF) consists of hydrogen-bonded planar TMA:DMF pairs, including a formamide-carboxyl C-H .. . O=C hydrogen bond. 2TMA:toluene (2TMA:Tol) contains centrosymmetric hydro gen-bonded TMA dimers; the packing of these dimers results in channels, in which the toluene molecules are disordered across centers of symmetry. Both potential planar conformations of TMA are observed in these structures; TM A and 2TMA:Tol contain the Z conformation, while the E conformation is obse rved in TMA:DMF. Computational studies suggest that Z-TMA is 0.96 kJ mol(-1 ) lower in energy than E-TMA, and that the rotational energy barrier is 46 kJ mol(-1). (C) 1999 Elsevier Science Ltd.