PDF Publication Title:
Text from PDF Page: 135
Speciation and Thermodynamics of Sulfur Vapor 125 Billmers and Smith [60] measured the UV-Vis absorption spectra of sul- fur vapor at various pressures (9–320 Torr) and temperatures (670–900 K). They attempted to evaluate the thermodynamic data of the equilibrium be- tween the molecules S3 and S4 using their broad absorption bands at 400 and 530 nm, respectively. The authors assumed that no other molecules con- tribute to these bands and that only one S4 isomer is present in sulfur vapor. Their reaction enthalpy of 78 kJ mol1 (at 800 K) for the conversion of 3 S4 into 4 S3 is in sharp contrast to the data in Table 1. For example, Rau et al. [24] determined this enthalpy as 128 kJ mol1 (at 298 K) and Detry et al. [31] obtained a value of 132 kJ mol1. However, if the 530 nm band is to be assigned to just one of two isomers of S4, the enthalpy determined by Bill- mers and Smith applies only to the reaction 3 S4(C2v)Ð4 S3 and not to the total amount of S4 in the vapor as studied by all previous authors using mass spectrometry. Furthermore, the enthalpy of 78 kJ mol1 is also not in agree- ment with the thermodynamic data obtained by high-level ab initio MO cal- culations which give a reaction enthalpy of 126 kJ mol1 at 700 K for the conversion of 3 S4(C2v) into 4 S3 (see Table 5). Hence, we conclude that the results of Billmers and Smith are seriously in error probably due to the high- er concentration of S3 compared to S4 in sulfur vapor and due to the much higher molar extinction coefficient of S3 which exceeds that of S4 by more than one order of magnitude! 2.7 Quantum-Chemical Calculations From the above discussion, it follows that the molecular composition of gas- eous and liquid sulfur is too complex to be elucidated fully by experimental techniques alone. Fortunately, theoretical methods have now developed to such a high level that reliable thermodynamic and structural information on sulfur molecules with up to 10 atoms may be obtained by ab initio MO cal- culations. Numerous theoretical calculations on Sn molecules have already been published but most of them can be considered outdated. Recent studies have shown that inclusion of a considerable number of polarization func- tions in the basis set as well as correction for electron correlation effects are essential not only in the single-point energy calculation but also in geometry optimizations. For this reason most of the empirical, semiempirical and ab initio MO calculations published before 1990 resulted in either incorrect ground-state structures of some of the treated Sn molecules or in incorrect relative energies of the various isomers, especially for n>3. These results will, therefore, not be discussed here. An exception is the theoretical treat- ment of the S3 molecule where the lowest-energy structure was correctly pre- dicted in 1986 to be of C2v symmetry on the basis of CASSCF and MR-CISD calculations while SCF and CISD calculations resulted in a global minimum of D3h symmetry [61]. This result was later confirmed by two other theoreti- cal investigations [50, 62]. The D3h structure is calculated to be 31– 44 kJ mol1 less stable than the bent open chain, depending on the method of calculation and the level of theory applied [46, 52].PDF 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)