TRANS-5-HYDROXYCYCLOOCTYLTRIMETHYL STANNANE: A STRUCTURE IN SPACEGROUP Cc DESCRIBED AS A DISPLACIVE MODULATION OF A P2₁/c PARENT STRUCTURE. A. C. Willis and A. D. Rae, Research School of Chemistry, Australian National University, Canberra, ACT, Australia, 0200

(CH₃)₃SnC₈H₁₄OH crystallises in spacegroup Cc with 16 molecules per unit cell, a 13.786(4) b 13.856(3) c 28.917(2) Å [[beta]] 100.47(1)deg.. An initial solution in spacegroup P2₁/c, Z = 4, used only the strong h even, k even reflections. This superimposes molecules separated by a/2 in the true structure. The Sn and 7 C atoms appeared as full weight atoms. The O and 4 remaining C atoms each appeared as a 1:1 disorder between sites separated by over 1Å. This was interpreted as two identical molecules related by a 120deg. rotation about the (CH₃)₃Sn - C bond. Ordering the disorder to create a model for the weak h odd, k odd relections allows 4 possible structures. This is predicted from the doubly degenerate irreducible representation associated with the modulation vector a*+b*. Local inversion centres are maintained and each structure may be described in terms of two c glide related sub-structures of C-1 symmetry. The coexistence of the substructures destroys the inversion as a symmetry element. Only half the c-glides of P2₁/c are retained, the choice between halves selects between equivalent structures that are inversion related. Within the substructures only half of the inversion centres of P2₁/c are retained and the choice between halves gives geometrically distinct options. Only the intensities of the weak extra reflections are sensitive to this choice, the wrong choice being unrefinable. The correct choice gave a sensible H-bonding network between O atoms while the other did not. Unconstrained refinement in spacegroup Cc creates difficulties for atoms that overlap in the average structure. This was overcome using features available in the Constrained Least Squares Refinement Program RAELS92. The 4 molecules in the asymmetric unit were constrained to have identical geometry that included a local mirror plane. This geometry was refined using local coordinates defined relative to refinable local orthonormal axial systems. Refinable TLX rigid body thermal parameterisation were used for each molecule, molecules inversion related in the substructure maintaining this relationship. The inversion symmetry of the substructures need not be maintained but departure from this was found to be minimal.