Trial models of the rigid molecule, spirodienone, were oriented and positioned in the crystallographic cell by means of visual packing analysis. Because there are two molecules in the asymmetric unit, 12 positional parameters were adjusted independently to obtain visually 'reasonable' packing arrangements. One such arrangement refined both by varying the 12 parameters with respect to repulsion energies as well as by full-matrix group refinement with respect to observed structure factors, which are reported elsewhere. The R values of the partially refined models were 42 % and 35 % respectively, at which point individual atom least-squares refinement was begun yielding a final R value of 4.8 %.

The structure of spiro [5,5]undeca-l,4,7,10-tetraene-3,9-dione, C~IHsO2, has been determined from three-dimensional X-ray counter data. The crystals have the space group P]" with a= 9.080 (4), b= 13.608 (2), c=7.493 (3)/~, a=91"11 (2), fl=96-91 (4), y= 100.50 (2) ° and four molecules per unit cell. The observed and calculated densities are 1.26 (1) and 1.272 g cm -a respectively. 3180 independent reflections were collected by the 0=20 scan method. The structure was solved independently by two methods. In one method a three-dimensional color television display system was used to search for the most likely packing arrangements in the unit cell. The other method involved an application of Boolean programming. Both methods arrived at the same model, which was refined by least-squares techniques to a conventional R index of 0"048 for the 1485 reflections having I>_ 3at. The two independent molecules in the asymmetric unit are virtually identical and consist of two cyclodienone rings joined by a common spiro C atom. The average C-C bond length for bonds involving the spiro C atom is 1"50 (1)/~. The average bond angle around this atom is 109"4 ° .