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122 Ralf Steudel et al. 2.4 More Mass Spectrometry In 1983 another study of the thermodynamics of reactions (1) appeared. Rosinger et al. [32] also used a combination of a Knudsen cell with a mass spectrometer to study the composition of sulfur vapor generated by evapo- ration of either elemental sulfur or mercury sulfide (HgS). Using an electron beam energy of only 12 eV the authors tried to suppress fragmentation pro- cesses as much as possible. The equilibrium constants and reaction en- thalpies for the equilibria (Eq. 1) were determined by the second and third law methods for the temperature range 430–490 K and were converted to 298 K using literature data for the entropies and heat capacities. The two sets of data obtained by Rosinger et al., given in Table 1, do not agree well with each other nor with the previously published reaction enthalpies, probably as a consequence of the small temperature range investigated. In particular, the enthalpies for n=6 and 7 deviate significantly from the values reported by the other authors. The previous values for S6 and S7 are supported by the corresponding reaction enthalpies derived for sulfur solutions and for liquid sulfur. In carbon disulfide solution 32 kJ mol1 are needed for the formation of S6 from 1 mol of S8, and 24 kJ mol1 for S7 [40]. In liquid sulfur at 116– 159 C, 30 kJ mol1 were obtained for the S6 formation from S8 and 24 kJ mol1 for S7 [13]. Thus, it is obvious that more work and new approaches are needed to es- tablish finally reliable thermodynamic data for the various equilibrium reac- tions in sulfur vapor. 2.5 Vibrational Spectra In 1988, it became obvious that the composition of sulfur vapor is even more complex than had been thought before: Lenain et al. published a Raman spectroscopic study of the sulfur vapor composition at temperatures of up to 700 C [20]. Signals for the cyclic species S8, S7, and S6 as well as for the acyclic molecules S4, S3, and S2 were observed and spectroscopic evi- dence for the presence of two conformational isomers of S4 and for either chain-like (Sn··) or branched-bonded species (Sn=S with n4) was presented. These authors came to the conclusion that the thermodynamic data of Rau et al. [24] are reliable except for the enthalpy of S7 formation from S8 which was said to be too high. The structure of S4 has been a puzzling problem for a long time since the infrared spectra, recorded after matrix isolation of quenched sulfur vapor [41–43], did not allow a convincing assignment to just one particular molec- ular structure, and theoretical calculations yielded different results depend- ing on the level of theory and the method applied. No fewer than six differ- ent structures have been reported by different authors to be the global mini- mum (see Scheme 1).PDF Image | Topics in Current Chemistry
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