A three-dimensional X-ray crystal structure analysis has been carried out on a complex formed from 2,3,11 ,I 2-dibenzo-l,4,7,10.13,16-hexaoxocyclo-octadeca-2,11 -diene (known as ' dibenzo-I 8-crown-6 ') and rubidium sodium isothiocyanate and the chemical formula [( Rbo.ss, Na, . , , ) NCS (C,oH,,O,J J established. The crystal also contains uncomplexed molecules of dibenzo-18-crown-6 which have the same bond lengths and angles as, but a different conformation from, the complexed molecules, There are four molecules of complex and two centrosymmetrical molecules of uncomplexed ether in a monoclinic unit cell having a = 19.322, b = 12.948. c = 13.001 8, p = 105.78 O , and space group P2,la. The structure was solved by direct methods, followed by Fourier and difference Fourier syntheses. All the atoms including hydrogen have been located ; full-matrix refinement converged with R 0-1 16. The six oxygen atoms of the complexed ether are coplanar and equidistant from the metal atom, with Rb-O(mean) 2-91 and Na-O(mean) 2.80 8. A hexagonal pyramid about the rubidium is completed by the nitrogen of the isothiocyanate (Rb-N 2.91 8) but the sodium is not in contact with the anion (Na-N 3.32 8 ) .

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The crystal structure of 1.8-dimethylnaphthalene has been solved by a reciprocal space-search method. Leastsquares refinement of 914 independent X-ray reflections measured on a diffractometer leads to R 0.080. The molecular distortions caused by steric interaction of the methyl groups have been analysed by means of strainenergy minimization calculations, which show that the repulsive energy between the methyl groups is reduced mainly by bond-angle distortion a t the junction between the methyl groups and the naphthalene nucleus. Crystals are monoclinic, space group P2,/c, Z = 4, a = 9.835(3), b = 7.01 2(3), c = 16.1 15(5) 8, p = 124.35(3)'. Research BV), Holland1~ recent years the techniques of calculating the minimum conformational potential energy of molecules have been further extended. For example, there is a strainenergy minimization procedure which enables all the independent molecular co-ordinates t o be varied simultaneously. This technique has been successfully applied to conformational problems in both organic and inorganic chemistry. However, the choice of suitable potential functions and the generality of this technique still poses some problems.The present combined X-ray and conformational study of 1,8-dimethylnaphthalene provides another opportunity to test potential functions and the minimization procedure, since molecular models and the related structure of 3bromo-1,8-dimet hylnapht halene ,3 suggest that there is appreciable steric interaction between the methyl substituents. E X PER 131 E : NTAL Tlie first was obtained froin a rather small crystal and proved adequate for solving the structure, but on refinement we found that these data did not define the structure with sufficient precision t o warrant a meaningful comparison with the results from the strain-energy minimization procedure. We therefore collected a second data set of better quality from a somewhat larger crystal in the manner described here.Slow evaporation of a solution of 1,8-dimethylnaphtIialene in acetone yielded transparent off-white single crystals. 1 'reliminary space-group data were obtained from precession photographs. A least-squares fit of 0, -0 values measured for several reflections using Mo-K, radiation gave the cell dimensions.Crystal Data.--C,,H,,, nl = 156.1. Monoclinic, a = !).835(3), b = 7.012(3), G = 16*115(5) A, p = 124*35(3)*, ?J = 917.53 k3, 2 = 4, D, = 1.13 g ~m -~. Space group P 2 J c from systematic absences: h01, 1 = 2n; OKO, ,' c. = 2n. Mo-K, radiation, A = 0-7107 A; ~(Mo-K,) = A suitable crystal of dimensions ca. 0.7 x 0.35 x 0.2 min was selected and mounted with gum arabic inside a Lindemann glass capillary. It was aligned along 001 on a threecircle Nonius automatic diffractonieter and the intensities of 9 14 independent reflections significantly above background with 0 < 25O were measured using 8-28 scans with zirconium-filtered Mo-K, radiation. Backgrounds were measured

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