Reversible sulfur compounds

Iron Sulfur Compounds. Many molecular compounds (18—20) are known in which iron is tetrahedraHy coordinated by a combination of thiolate and sulfide donors. Of the 10 or more stmcturaHy characterized classes of Fe—S compounds, the four shown in Figure 1 are known to occur in proteins. The mononuclear iron site REPLACE occurs in the one-iron bacterial electron-transfer protein mbredoxin. The [2Fe—2S] (10) and [4Fe—4S] (12) cubane stmctures are found in the 2-, 4-, and 8-iron ferredoxins, which are also electron-transfer proteins. The [3Fe—4S] voided cubane stmcture (11) has been found in some ferredoxins and in the inactive form of aconitase, the enzyme which catalyzes the stereospecific hydration—rehydration of citrate to isocitrate in the Krebs cycle. In addition, enzymes are known that contain either other types of iron sulfur clusters or iron sulfur clusters that include other metals. Examples include nitrogenase, which reduces N2 to NH at a MoFe Sg homocitrate cluster carbon monoxide dehydrogenase, which assembles acetyl-coenzyme A (acetyl-CoA) at a FeNiS site and hydrogenases, which catalyze the reversible reduction of protons to hydrogen gas. 

This reaction is the reverse of the initial ketyl radical formation by the benzophenone triplet and is therm Q4ynamically favorable. The experiments using optically active alcohols as source of hydrogen atoms show, however, that under normal conditions this reaction is unimportant. This is probably due to other, more efficient pathways for reaction of the ketyl radicals or perhaps to diffusion rates which separate the radicals before reverse transfer can occur. That this reaction can be important in some cases even without the presence of sulfur compounds was shown by studying the photoreduction of benzophenone in optically active ethers.<68) Although the reaction of benzophenone in methyl 2-octyl ether is only 0.17 times as fast as that in isopropanol, ethers can be used as sources of hydrogen atoms for photoreduction ... 

One sulfur compounds, X, is known which becomes reversibly colored and paramagnetic (even in the dark) in toluene at 80°.180 It is used in rubber processing. 

The number of bisylides, in which the carbon atom is stabilized by two neutral sulfur compounds, is restricted so far to one example, C(S NMe Ph2)2, with sulfur (IV). The proton of the conjugated acid (HC(S NMe Ph2)2) can be reversibly removed on an ion exchange resin loaded with OH protonation is achieved with 10% HCIO4 in methanol as shown in Fig. 8. The bent structure with an S-C-S angle of 117° and bond shortening of the C-S bond upon deprotonation of the related cation proves this compound as a typical carbon(O) compound stabilized by the Lewis base S(NMe)Ph2. Although the compound was reported to be stable even... 

Again (as mentioned in Section V,C) sulfur compounds perform better than CO, as can be seen in Fig. 20, because they are better dehydrating agents. When Cr/silica is reduced by COS or CS2 a black chromium sulfide forms. Reoxidation then converts it back to the hexavalent oxide. The catalyst retains no sulfur, but it often takes on a new reddish hue and the activity is greatly improved. This is probably an extension of the trend already observed in Fig. 10, which shows both activity and termination to increase as the catalyst is dehydrated. Perhaps the color change from yellow to orange, and finally to red for sulfided catalysts, indicates a transition from chromate to dichromate, or maybe just less coordination to hydroxyls. Adding water vapor to a sulfided catalyst completely reverses the benefit. 

Carbon Dioxide Condensation, Sulfurous Compound Absorption. Carbon dioxide is condensed by cooling the gas from its dew point to about -55°C the condensate is contaminated with sulfurous compounds. Substantial amounts of carbon dioxide can be condensed if the dew point is relatively warm. A synthesis gas at 1000 psia with 30 mol % carbon dioxide has a dew point of about -30°C, and approximately 65% of the carbon dioxide condenses to a liquid in cooling to -55°C. Removal of carbon dioxide by condensation reduces the amount which must be removed subsequently by absorption. Condensation is preferred over absorption because it is more reversible and hence is more energy efficient and less capital intensive. 

Figure 2. Radioactivity chromatogram of sulfur compounds derivatized with monobromobimane. The reversed-phase HPLC separation is based on the hydrophobic properties of the bimane-sulfur adducts but peak area is based on "S-radioactivity of the compounds. At time 0 sulfite and thiosulfate impurities are present before addition of the hepatopancrease tissue homogenate. This was a 60 min experiment to determine the sulfide detoxifying functions of the hepatopancrease of the hydrothermal vent crab Bythograea thermydron. During this time the proportion of radioactivity in sulfide rapidly decreases and thiosulfate and sulfate accumulate as end products. Two intermediates, pi and p2 accumulate then decrease during the experiment. The two intermediates are believed to be polysulfides based on similar elution times of polysulfide standards. (Figure is the unpublished chromatograms from the data in Vetter et al. (24)-) continued on next page.

Temperature Measurements. Sodium line reversal temperature profile measurements were made on the flame series with varying additions of H2S. Results for H2/O2/N2 (3/1/4,5,6) are shown in Figure 3. The increase in temperature with distance above the burner is due to the slow recombination of the radicals H and OH. In the stoichiometric flames the temperature reaches a plateau in a few centimeters above the burner. In the richer flames the temperature gradient is steeper indicating a larger departure of the radical concentration from equilibrium values. The equilibrium temperatures decrease with H2S addition. However, the presence of sulfur compounds enhances radical recombination (6,11) producing almost equivalent temperature profiles, independent of H2S addition. 

The activity of sulfur towards platinum complexes has led to investigation of so-called rescue agents to ameliorate the side effects of platinum therapy, without compromising its anti-tumor activity. These nucleophilic sulfur compounds include sodium thiosulfate (STS), sodium diethyldithio-carbamate (Naddtc), (S)- 2-[(3-aminopropyl)amino]ethyl phosphorothioic acid (WR-2721, Ethyol , amifostine), glutathione (GSH), methionine, thiourea, cysteine, -acetylcysteine, penicillamine, biotin, sulfathiazole, sodium 2-mercaptoethanesulfonate (mesna), and its dimer (di)mesna (BNP-7787). The protective effect of these compounds is either due to prevention, or reversal of Pt-S adducts in proteins. Some of the more promising of the above-mentioned compounds (see Fig. 1) will be discussed below. 

The interaction of H2S, organic sulfides, and other sulfur compounds may involve a number of consecutive steps including reversible molecular adsorption of the sulfur compound, its dissociation, reorientation or reconstruction of the metal surface, formation of a 2-D surface sulfide, and at still higher H2S/H2 ratios, formation of a three-dimensional (3-D) (bulk) metal sulfide. Kinetic information about these processes may generally be helpful in elucidating the adsorption mechanism. Unfortunately, such quantitative kinetic information is not adequately available, with one exception, formation of bulk sulfides (9, 96). 

Sulfur oxides (S02 and S03) present in flue gases from upstream combustion operations adsorb onto the catalyst surface and in many cases form inactive metal sulfates. It is the presence of sulfur compounds in petroleum-based fuels that prevent the super-sensitive base metal catalysts (i.e., Cu, Ni, Co, etc.) from being used as the primary catalytic components for many environmental applications. Precious metals are inhibited by sulfur and lose some activity but usually reach a lower but steady state activity. Furthermore the precious metals are reversibly poisoned by sulfur compounds and can be regenerated simply by removing the poison from the gas stream. Heavy metals such as Pb, Hg, As, etc. alloy with precious metals and permanently deactivate them. Basic compounds such as NH3 can deactivate an acidic catalyst such as a zeolite by adsorbing and neutralizing the acid sites. 

Two methods for the formation of aldehydes embody C—C bond formation and then its cleavage by a reverse aldol reaction after reduction, and a third involves similar formation and breakage of a C—P bond. Reaction of esters with the sulfur compound (55 Scheme 26) affords ketones (56) which can be reduced with NaBHa at 0 °C to the alcohols (57). Simply warming this alcohol, or better, treating it with aqueous potassium carbonate, gives the aldehyde in about 95% yield for aromatic compounds but somewhat less for aliphatic dialdehydes can be obtained from diacids.Acylmalonates (58 Scheme 27)... 

When the hrst photosynthetic sulfur-compound oxidizers hrst appeared, the development of full sulfureta would have been possible. Sulfate reducers would take sulfate from the external environment and eventually produce H2S. Then the photosynthetic oxidizers would reverse the steps, e.g.,... 

Organic sulfur compounds are also able to form reversible redox systems. Mercaptans (monosulfides) are oxidized to disulfides according to... 

The wash-out of reduced sulfur compounds was neglected owing to the small solubility in water of these species. Furthermore it was assumed that the dry deposition of reduced compounds is negligible as compared to the strength of chemical sinks. Thus, Judeikis and Wren (1977) demonstrated by laboratory studies that the dry deposition velocity of DMS and H2S on selected soil samples ranges from 0.015 to 0.28 cm s 1. They mention, however, that these values are likely to be upper limits due to a possible reversible physical adsorption. 

Tracer studies with l4C and 35S also provided information about the mechanism of the overall reaction given by Eq. (23) (233). Experiments employing reaction mixtures containing all four species or only carbon or sulfur compounds, with steam as a diluent, were carried out with labeled l4C02 and 35S. It was possible to measure the reverse l4C transfer when sulfur compounds were removed from the reaction chamber. Thus the exchange reaction... 

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