Topics in Current Chemistry

PDF Publication Title:

Topics in Current Chemistry ( topics-current-chemistry )

Previous Page View | Next Page View | Return to Search List

Text from PDF Page: 026

16 Ralf Steudel · Bodo Eckert Irradiation at low temperatures is another method to convert a-S8 into polymeric sulfur [60]. Initially, the irradiation of a-S8 with visible light (l<420 nm) or UV radiation at temperatures of 2–70 K produces free radi- cals by homolytic dissociation of S-S bonds as demonstrated by electron spin resonance (ESR) spectroscopy [61]. On warming to room temperature the ESR signals fade away since the radicals decay by recombination as well as by triggering a ring-opening polymerization resulting in the formation of polymeric sulfur [60]. 2.2 Molecular and Crystal Structures The following section provides a brief summary of the molecular and crystal structures of the solid allotropes of sulfur mentioned in the Introduction. More specific details about the structures of most of the allotropes can be found in the cited literature. A conclusion concerning the characteristics of the molecular as well as of the crystalline structures of sulfur will be drawn at the end of this section. There are no crystalline forms known for the low atomic molecules S2 to S5 although these molecules are present in the gaseous and liquid phase [49, 59]. Since the cyclic molecules S16, S17, S19, and Sn (n􏱤21) have not yet been prepared, no molecular and crystal structure data are available. However, a mixture of large sulfur rings Sx(􏰵25) was observed as an unstable resi- due during the preparation of S12 (see above) [43]. The Raman spectrum of this mixture resembles that of a high pressure amorphous sulfur form as well as that of polymeric sulfur, often called S􏰺 (see the section on high-pres- sure forms of sulfur below). Sulfur atoms are well known for their pronounced tendency of catenation. Since the sulfur atom has an s2p4 outer shell electronic configuration in the ground state the sulfur-sulfur bonds can formally be built from the two un- paired electrons in the 3p orbitals in which case the optimum value of the bond angle would the be 90􏰹. The observed bond angles of ca. 106􏰹 can be explained by the repulsion of the non-bonded sulfur atoms or by mixing some s-character into the covalent bonds (s-p hybridization). The two re- maining electrons on the 3p level are non-bonding and occupy an orbital of local p-symmetry perpendicular to the neighboring S-S bonds. The repul- sion of the lone-pairs of adjacent S atoms should lead to a dihedral angle of 90􏰹 at the S-S bonds in the case of a “free” chain corresponding to a mini- mum of configurational energy [54]. This model explains why chains of cu- mulated S-S bonds exhibit a three-dimensional zig-zag conformation rather than a planar configuration. In consequence, a large number of quite differ- ent cyclic and chain-like molecules is, in principal, possible. Ring closure of a sulfur chain usually causes deviations of the bond angle as well as of the dihedral angle from the ideal values due to interaction of the lone-pairs of the next-nearest S atoms. These peculiarities can be easily understood, for example, if going from simple molecules like disulfane H2S2 to more com- plex molecules with cumulated sulfur bonds, especially sulfur rings [62].

PDF Image | Topics in Current Chemistry

PDF Search Title:

Topics in Current Chemistry

Original File Name Searched:

Elemental-Sulfur-und-Sulfur-Rich-Compounds-I.pdf

DIY 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)