The structure of a new heteroaromatic sulphur compound has been determined; the results support the inclusion of d-orbitals in CNDO molecular orbital calculations.ALTHOUGH derivatives of 2,2'-bi-l, 3-dithiole have been examined, it is only recently that Wudl et aZ.1 have isolated the parent compound itself which being an odd-membered sulphur-containing heterocycle, is expected to be a planar aromatic system, similar to the sulphur-containing systems examined by Visser et aZ.2 We have determined the crystal structure of 2,2'-bi-1,3-dithiole and compared the results with predictions based on CNDO calculations.2,2'-Bi-1,3-dithiole forms orange, light-sensitive needles; crystal data: C,H4S,, monoclinic, a = 7.364, b = 4.023, G = 13.922 A, = 101.42*, 2 = 2, space-group P2,/c, p = 10.77 cm-l. 2496 independent hkl intensities were collected on an automated Picker full-circle diffractometer with Mo-K, (A = 0.71069 A) radiation. The structure was solved by the standard heavy-atom method, and the model, including the hydrogen atoms, was refined by full-matrix least-squares to a final R of 0.048 on the observed data.

The three-dimensional crystal structure of d-(+)-biotin, the coenzyme responsible for the fixation and transfer of carbon dioxide in biological systems, has been determined by x-ray diffraction techniques in two laboratories independently. The results of the two determinations are compared and discussed in terms of the mode of nucleophilic activation of the coenzyme. Of particular relevance may be the participation of the ureido carbonyl oxygen as acceptor of a strong hydrogen bond ( 0 . . . O distance 2.54 A) which may cause the observed lengthening of the ureido carbonyl bond to 1.25 8, and shortening of the carbonyl carbon-nitrogen bonds to 1.33 and 1.35 A. These results suggest a partial delocalization of the electronic charge within the ureido group, and are supportive of a proposed activation via polarization mechanism. A close intramolecular nonbonded contact involving the nitrogen atom proximal to the valeryl chain probably precludes carboxylation from that side of the molecule.Biotin (I) participates as coenzyme in a variety of carboxylase, decarboxylase, and transcarboxylase systems. Its role is to fix CO2 for eventual transfer, and it accomplishes its task in a two-step fashion during which a labile enzymebiotin-COz complex is formed.2 Early crystallographic 02'II studies of biotin by Traub3 confirmed the relative stereochemistry a t the asymmetric carbons previously suggested by Grob and von S p r e~h e r .~ In studies of the biotin carboxylation mechanism Bonnemere, Hamilton, Steinrauf, and Knappej prepared the bis-p-bromoanilide derivative of the major product in the biotin methyl ester-methyl chloroformate reaction and, by crystallographic determination, confirmed that the primary site of carboxylation in this model reaction was the ureido 1'-N nitrogen. Knappe had already shown that the major product had identical properties with the dimethyl ester of carboxybiotin obtained by diazomethane trapping experiments in biological systems. Subsequently Trotter and Hamilton6 measured the anomalous dispersion properties of the same compound, 1 '-N-carboxybiotin bis-p-bromoanilide, and established the absolute configuration of biotin.Continued interest in the mode of action of biotin-dependent enzymes as well as our own interest in careful structural investigation of the biotin vitamers prompted a reinvestigation of the biotin crystal structure, and ultimately led to its determination by two groups working independently of the other. A collaborative report of our results is presented. Where necessary to distinguish between the results of the two groups, work performed by DeTitta and Edmonds, Medical Foundation of Buffalo, is designated I, and by Stallings and Donohue, University of Pennsylvania, 11. Experimental SectionI. Needle-shaped crystals elongated about the a axis were grown by slow cooling of an aqueous solution of biotin (Sigma Co. Lot no. 12C-2000) saturated at 95 'C. A crystal of cross section 0.05 X 0.14 mm and length 0.88 mm was cleaved from a longer crystal and mounted about the a axis for da...

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Neutron diffraction studies of single crystals of hexamethylbenzene at 298 K and at 130 K indicate that the molecule in phase I1 has approximate D3d symmetry. The amplitudes of libration of the methyl group and of rigid body motions of the molecule are consistent with earlier data, except that the barrier to methyl group rotation appears to be lower by about 0.5 kcal/mol (2100 J/mol).Consideration of intra-and inter-molecular hydrogen atom contact distances and calculated potential energy curves using a 6-exp potential function suggest that intermolecular forces are important in determining the barrier to rotation of the methyl groups and that substantial changes in the intermolecular packing must be responsible for the lambda-point transition at 11 6 K and the consequent profound change in the potential barrier to internal rotation which has been previously observed.* As is usual for the correction for riding, an overcorrection is made if the motion is very large.

Powder diffraction profiles of well crystallized compounds can be fitted to distributions of the type Iθ=I°(1 + k2x2)-n where k is a scale factor related to the half width of the profile. The value of n varies with the diffraction angle, 2θ, and is generally different for the low-angle and high-angle sides of the same profile. Limiting values of n for a specific Guinier camera-micro-densitometer combination are 1.2 ≤ n ≤ 2.3. Similar values are obtained for diffracto meter profiles after Ktt2 stripping. Line broadening due to departure from perfect crystallinity in the specimen affects the value of n a swell as that of k.The above observations are interpreted interms of the convolution of a Gaussian with a Lorentzian distribution, the exponent n of the convolute being dependent upon the relative half widths of these two functions, expressed as the ratio bL/bG.