PDF Publication Title:
Text from PDF Page: 137
Speciation and Thermodynamics of Sulfur Vapor 127 Scheme 2 sulting in a pressure ratio of S8:S9:S10=1:0.020:0.004 (equal to the molar ra- tio). Lower fugacities will of course reduce the concentrations of S9 and S10 in the vapor. This result is in fair agreement with the composition of the equilibrium sulfur melt which, after quenching in liquid nitrogen, extraction and HPLC analysis, contains ca. 54 mass % of S8, 0.6% of S9 and 0.2% of S10 at 300 C, equivalent to a molar ratio of 1:0.010:0.003 [13]. Thus, S9 and probably also S10 should be detectable by mass spectrometry in saturated sulfur vapor, which has in fact been demonstrated [28]. The infrared spectra of quenched sulfur vapor exhibit an absorption at 683 cm1 which has tentatively been assigned by several authors to a branched sulfur chain or a ring of the type Sn=S (n>4) with one exocyclic atom [43, 66]. In search for plausible isomers of S8, molecular-dynamics density-functional calculations had shown that the energy of a cluster-like S8 isomer of C2 symmetry is only 42 kJ mol1 higher than that of cyclo-S8 [67]. This isomer has a bicyclic structure and is only loosely related to a branched ring; its structure is shown in Scheme 2 (bond lengths in pm). Our recent high-level ab initio MO calculations by the G3X(MP2) method placed this isomer only 33 kJ mol1 (DH298) higher than the crown-shaped S8 ring [68]. This means that 1% of all S8 molecules in sulfur vapor at the boiling point (445 C) will be present as isomers of C2 symmetry. Further- more, these calculations demonstrated that several other isomers of S8 exist with relative energies between those of the C2 structure and the diradical chain which also has a C2 symmetry. Some of these species exhibit SS stretching fundamentals in the 600–700 cm1 region. For instance, the iso- mer S7=S with a structure similar to that of the related homocyclic oxide S7=O [69] is by 93 kJ mol1 less stable than the S8 crown if the exocyclic sul- fur atom is in an axial position but by 96 kJ mol1 if this atom is in an equa- torial position with respect to the ring (DH298 values) [68]. The formation of an S7=S species was recently “observed” in a molecular- dynamics simulation of the reactions in liquid sulfur at 400 K [70]. It was found that some of the S8 ring molecules homolytically open up on excita- tion of one electron from the HOMO to the LUMO. The chain-like diradicals Sn·· thus generated partly recombine intramolecularly with formation of S7=S rather than cyclo-S8. The S7=S molecule may then react with another Sn·· diradical to form a chain of 16 atoms. Such chains are, of course, alsoPDF Image | Topics in Current Chemistry
PDF Search Title:
Topics in Current ChemistryOriginal File Name Searched:
Elemental-Sulfur-und-Sulfur-Rich-Compounds-I.pdfDIY PDF Search: Google It | Yahoo | Bing
Sulfur Deposition on Carbon Nanofibers using Supercritical CO2 Sulfur Deposition on Carbon Nanofibers using Supercritical CO2. Gamma sulfur also known as mother of pearl sulfur and nacreous sulfur... More Info
CO2 Organic Rankine Cycle Experimenter Platform The supercritical CO2 phase change system is both a heat pump and organic rankine cycle which can be used for those purposes and as a supercritical extractor for advanced subcritical and supercritical extraction technology. Uses include producing nanoparticles, precious metal CO2 extraction, lithium battery recycling, and other applications... More Info
CONTACT TEL: 608-238-6001 Email: greg@infinityturbine.com (Standard Web Page)