A detailed X-ray structure analysis of thianthren, C,,H,S,, has been made.The molecule is folded on the S-S line, so that the two benzene rings lie in two planes a t an angle of 128". The sulphur valency angle is 100" and the C-S bond length is 1-76 0.01, A. THIANTHREN (diphenylene disulphide), C,,H8S, (I), has long been considered (although on circumstantial rather than direct evidence) to be an example of a folded molecule in which the folding enables the central sulphur atoms to retain their '' natural " valency angles even though they form part of a ring system. The folded configuration was first suggested by Bergmann and Tschudnowsky to account for the dipole moment of thianthren (about 1+ D). Attempts to substantiate this suggestion by the methods of classical stereochemistry Numbering used in this paper.Chemical numbering.(for a summary see Turner and Harris 2, were not successful, but this was attributed to the flexibility of the molecule permitting the folding to be reversed easily. X-Ray measurements on crystalline thianthren were first made by Prasad, Shanker, and Peermohamed and by Cox; showed that the X-ray intensities and optical properties were consistent with a structure built up of layers of non-planar molecules, and they estimated the angle of fold to be about 140".The present work was undertaken to obtain accurate values of the sulphur valency angle and the C-S bond length; the latter is of interest since, as Sutton has pointed out, the sulphur atoms could use dx as well as jm orbitals and conjugation could therefore occur even though the molecule is not flat.
Wood and Crackston
EXPERIMENTALSuitable crystals were grown by evaporation from acetone solution in the form of radiating clusters of prisms, elongated along the b-axis and exhibiting the forms {OOl}, {TOl}, {SOl], (Oll), (021}, {110}, and {ill}. The unit-cell dimensions, measured from Straumanis photographs, are a = 14.484 f 0.002, b = 6.147 f 0.002, c = 11.932 f 0.002 A, and p = 109" 51.6' f 1-5'. These results, which agree with those of Wood and Crackston, are also in accord, when suitably transformed, with the axial ratios given by Friedel and Crafts.' It should be noted that in Friedel and Crafts's description of the crystals, as transcribed by Groth,B two planes are wrongly indexed as (101) and (1 11) ; on Groth's orientation and axial ratios they should be (201) and (221) respectively. The density determined by flotation is 1-44 g. cm.-3 (calc. 1-44, by assuming four molecules in the unit cell). The systematic absences of (hOE) reflections for h odd and (OkO) for R odd indicate unambiguously that the space group is P2Ja -Cesh. An optical examination in sodium light gave the following values for the principal refractive indices and the optic axial angle, in good agreement with those of Wood and Crackston : 5 a = 1-65, p = 1.76 (parallel to [b]), y = 1.95 (inclined at 69" to [c] in the obtuse angle p), and 2V = 82".
