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132 Ralf Steudel et al. 3 Summary The molecular composition of sulfur vapor is much more complex than had been thought by all authors before the year 1990. Not only the molecular size can vary between 2 and—at least—10, but there are cyclic and chain-like isomers as well as branched rings and chains and even clusters to be taken into account. This makes experimental investigations rather difficult. How- ever, the reaction enthalpies calculated by the most sophisticated ab initio MO methods (Table 5) are in good agreement with the most reliable experi- mental data obtained by mass spectrometry and vapor pressure measure- ments (Table 1, column 5). While the structures of S2 (D4h) and S3 (C2v) are well known, there are at least two S4 isomers in sulfur vapor: the green light absorbing ground state cis-planar structure of C2v symmetry with a characteristic infrared absorp- tion at 662 cm1 and the red light absorbing isomer characterized by an in- frared band at 642 cm1 (both in Ar matrix) [43]. On the basis of recent high-level ab initio MO and DFT calculations (see Table 3), the latter isomer is reassigned as the trans-planar form with C2h symmetry (see Scheme 1). Since these two isomers occur mainly under non-equilibrium conditions their energy difference of 41 kJ mol1 may not be an obstacle. In equilibrium sulfur vapor, the absorption at 520 nm is much stronger than the absorption at 600 nm, i.e., the C2v form of S4 is the dominating isomer. This finding is also supported by the excellent agreement between the calculated and mea- sured enthalpies of formation of S4 from S8. The structures of all Sn species larger than S4 are cyclic in sulfur vapor. Acknowledgements This work has been supported by the Deutsche Forschungsgemein- schaft, the Verband der Chemischen Industrie and the National University of Singapore. References 1. Review: R. Steudel, Chemie unserer Zeit 1996, 30, 226. 2. M. Kitto, Sulphur 2001, 275, 30. 3. Review: R. Steudel, B. Eckert, Top Curr. Chem. 2003, 230, in print. 4. J. Donohue, The Structures of the Elements, Wiley, New York, 1974, pp. 324. 5. Review: R. Steudel in: Sulfur - Its Significance for Chemistry, for the Geo-, Bio- and Cosmosphere and Technology (A. Müller, B. Krebs, Eds.), Elsevier, Amsterdam, 1984, pp. 3–37. 6. J. Steidel, J. Pickardt, R. Steudel, Z. Naturforsch. Part B 1978, 33, 1554. 7. R. Steudel, J. Steidel, J. Pickardt, F. Schuster, R. Reinhardt, Z. Naturforsch. Part B 1980, 35, 1378. 8. P. Coppens, Y. W. Yang, R. H. Blessing, W. F. Cooper, F. K. Larsen, J. Am. Chem. Soc. 1977, 99, 760; S. J. Rettig, J. Trotter, Acta Cryst. C. 1987, 43, 2260. 9. R. Steudel, K. Bergemann, J. Buschmann, P. Luger, Inorg. Chem. 1996, 35, 2184. 10. R. Steudel, J. Steidel, R. Reinhardt, Z. Naturforsch. Part B 1983, 38, 1548. 11. H. Luo, R. G. Greene, A. L. Ruoff, Phys. Rev. Lett. 1993, 71, 2943. 12. V. V. Struzkhin, R. J. Hemley, H. Mao, Y. A. Timofeev, Nature 1997, 390, 382.PDF Image | Topics in Current Chemistry
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