Polysulfides electronic structure

E24.25 The blue colour of the solution is due to the presence of anionic radical Sj , identical to that found as 83" in lapis lazuli described in Section 24.15. The cause of colour is the excitation of the unpaired electron in the electronic structure. In solution polysulfide, radical anions are very sensitive to O2 from the atmosphere upon oxidation the colour is lost. 

In polysulfide solutions there exist chainlike dianions with practically arbitrary chain lengths n. Berghof, 8ommerfeld, and Cederbaum have investigated the onset of stabihty in the 8 (n=2-8) series of dianions based on 8CF calculations with the DZPD basis set [89]. The isolated chainlike 8 isomers were found to exhibit twisted structures, and the onset of electronic stabihty (with respect to electron auto detachment) was predicted to occur at 71=7. Branched isomers were found to be electronically more stable than the... 

The molecular structures of the isolated polysulfide monoanions 8 with n=2-9 have been studied by density functional calculations and those of the smaller ions also by ab initio MO calculations. Compared to the neutral 8 molecules the extra electron occupies an antibonding orbital resulting in longer 88 bonds. The species 83 is bent and of C2V symmetry (a=115°) [140, 141]. 84 was calculated to be a planar ion of C2V symmetry (similar to the neutral molecule 84) but the planar C2h structure is only slightly less stable [140, 141]. The most stable isomer of 85 is a chain of Q symmetry sim-... 

The increasing importance of hypervalent 3-centre 4-electron bonding on going down Group 16 is reflected in the structures of Sej anions with n = 9-11, which are without parallel for the polysulfides. Although a chain structure is still apparent for the nonaselenide anion of [Sr(15-crown-5)2]Se936 (Figure 5a),... 

Nickel (continued) polysulfide complex, 31 98 half chair conformation, 31 115 porphyrin complexes, 32 13 in proteins, 47 284-285 quadruply bridged dimers, 40 211-214 steric structures, 40 190-194 quaternary phosphonium salts of, 6 31-32 redox chemistry probes of structure, 32 243-245 steric and electronic requirements, 32 242-243... 

Sulfur forms a series of homoatomic dianions catena-S (x = 2-8), which, without exception, have unbranched chain structures in the solid state.The electrochemical reduction of cyclo-Sg in aprotic solvents occurs via an initial two-electron process to produce catenaS P In solution, catena- and other long-chain polysulfides, e.g. catena- and catenaSi, dissociate via an entropy-driven process to give radical anions S (x = 2-4), including the ubiquitous trisulfur radical anion (x = 3). This intensely blue species is the chromophore in the mineral lapis lazuli, which is used in the manufacture of jewellery. 

Thus, sulfide and polysulfides react as Lewis bases to transfer an electron pair from a pz orbital to the vacant dihalogen inner-sphere process (Fig. 3). In Lewis structures, the transient intermediate I should be formed ... 

Structure determinations by X-ray and electron diffraction methods have demonstrated the presence of unbranched chains of up to six divalent sulfur atoms in inorganic polysulfides, four divalent ones in salts of polythionic acids, and three in disulfonyl, dialkyl, and cyanogen derivatives. The synthetic and Raman-spectroscopic work of Feh6r has established the presence of unbranched structures with up to five sulfur atoms in organic polysulfides, and eight in the cyanogen polysulfide, polysulfur dichloride, and hydrogen polysulfide series. The results are supported by extensive physicochemical evidence of less conclusive nature. 

The stereochemistry of organic sulfur compounds was reviewed very extensively by Laur in 1972 and that of organic polysulfides by Rahman et in 1970. Since those days, however, considerable progress has been made. The molecular and crystal-structures of more than 60 cyclic and acyclic polysulfanes of type R-S -R have been determined by X-ray diffraction on single crystals. In rare cases, electron diffraction of the vapor of R-S -R molecules has been used to determine the structure (see Diffiaction Methods in Inorganic Chemistry). In addition, the structures of several acyclic polysulfane oxides such as RS-SO-SR and RS02-S -S02R (n = 1, 2, 3), and of the trisulfane cation (MeS)3+, have been determined. 

As seen from its amino acid sequence, the catalytic subunit PsrA of polysulfide reductase belongs to the DMSO reductase family of molybdo-oxidoreductases [26,29]. The structure of several single-subunit enzymes of this family is known. As a rule, the molybdenum ion coordinated by two MGD molecules appears to be the electron donor or acceptor to the respective substrate in these enzymes. A cavity extending from the surface of the protein to the molybdenum close to its center is seen in all structures. The substrates probably reach the active site near the molybdenum through this cavity, and the products are released at the surface through the cavity. With the assumption that PsrA... 

Elastomer-modified epoxy resins are used in composites and structural adhesives, coatings, and electronic applications. Similar approach to toughen epoxy vinyl esters using other elastomeric materials has been reported (204). Other elastomer-modified epoxies include epoxy-terminated urethane prepolymers, epoxy-terminated polysulfide, epoxy-acrylated urethane, and epoxidized polybutadiene. Preformed dispersions of epoxy-insoluble elastomers have been developed and reported to achieve toughening without Tg reduction (205,206). 

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