MONOCLINIC THIOUREA INCLUSION COMPLEXES: IMPLICATIONS OF THE DURENE ADDUCT CRYSTAL STRUCTURE. John S Rutherford, Sultan Qaboos University, Sultanate of Oman, Emmanuel Marfo-Owusu, University of Transkei, South Africa and Peter S Ford and Judith A K Howard, University of Durham, UK.

The crystal structure of the durene (1,2,4,5-tetramethylbenzene) channel inclusion complex with thiourea, containing 11 molecules in the asymmetric unit, was determined using single crystal diffractometer data at 18^oC, and refined to an R Factor of 0.104. The repeat distance along the channel is 37.33 A, which corresponds to 4 durene molecules end-to-end, and to 3 complete turns of the thiourea chains. These two sublattices are each remarkably regular, constituting a lock-in commensurate superstructure in terms of the Rennie-Harris model [J. Chem. Phys., 96, 7117 (1992)]. The oblong cross-section of the durene molecules, as viewed along the channel axis, is accommodated by a herringbone arrangement of neighbouring channels. This effect is produced from the normally regular honeycomb host lattice through an antiparallel displacement of alternate sheets of thiourea molecules. Since the space group of this adduct is related to the rhombohedral aristotype through the following subgroup relations:

R3(bar)c A2/a P2₁/a P2₁/n

I(1) II(3) III(6) IV(18)

(subgroup orders in parentheses), it can be deduced that monoclinic thiourea host lattices correspond to space group III, which actually occurs for the trans-1,2-dibromocyclohexane adduct. A phase transition directly from I to III is also consistent with the observed behaviour of the cyclo-hexane adduct at low temperatures [J. Solid State Chem., 10, 46 (1974)]. The intensity statistics expected for such lock-in superstructures will be discussed, as will possible extensions to the Rennie-Harris model.

[Crystal data: 2C₁₀H₁₄.9SC(NH₂)₂, monoclinic, space group P2₁/n, a = 18.595(4) A, b = 15.324(3) A, c = 18.780(4) A, b = 105.07(3)^o, Z = 4].