Sulfur, polymorphism

A number of lower sulfur oxides have been described. Most of these oxides are derived from cyclic sulfur polymorphs and were usually prepared by oxidation of these molecules by organic peroxo acids. The oxides have the general formula SraO and n may vary from 5 to 10. For n = 7 even the dioxide S702 is known.4 Not all of these phases were characterized by X-ray diffraction, but the molecular structures are certain with respect to vibrational spectroscopy. The oxygen atom is in exo position with respect to the sulfur ring as it has been shown by X-ray diffraction for SgO and S70, respectively (Figure 2).5,6... 

Selenium and tellurium have the A8 helicoidal zigzag chain structure at ambient conditions (1). Both show maxima in their melting curves at about 50 kb at 963°K for selenium (32) and at an average p-T of about 12 kb at 743°K (22, 29, 33, 34, 35) for tellurium. These maxima are suspected to be related to the maximum in the melting curve of sulfur at 86 kb at 953°K (12) because at these conditions the sulfur polymorph... 

Do bacteria oxidize the diflFerent sulfur polymorphs at the same or at diflFerent rates An answer to this question could be important in the manufacture of degradable plastics or controlled release fertilizers. 

The mechanism of the Claus reaction is complicated, and the sequence of the surface reaction steps is not fioUy elucidated (361). The primary step is certainly the strong adsorption of SO2 on acid—base pairs with formation of sulfite and bisulfite species (354—356), which later react with gaseous or weakly adsorbed H2S. The strength of the SO2 adsorption depends on the surface basicity, which is influenced by the amount of sodium present (225) however, it must be taken into account that the reactivity of the formed sulfites is lower on more basic surface sites. Thus, the acid—base properties of the catalyst play a key role in this reaction. According to Clark et al. (362), sulfate and thiosulfate species apparently form in addition to the sulfites. It was proposed that sulfate and thiosulfate react to form [HS404] ions, which then react with H2S to form the S3 sulfur polymorph, which is subsequently converted into cychc Se and Sg molecules. 

We now consider other homocyclic polymorphs of sulfur containing 6 - 20 S atoms per ring. A rhombohedral form, t-sulfur, was first prepared by M. R. Engel in 1891 by the reaction of concentrated HCl on a saturated solution of thiosulfate HS2O3 at 0°. It was shown to be... 

The ring adopts the chair Ibrm and its dimensions are compared with those of other polymorphs in Table 15.5. Note that cyc/o-S has the smallest bond angle and dihedial angle of all poly-sulfur species for which data are available and this, Uigether with the small hole at the centre of the molecule and the efficient packing within the crystal, lead to the highest density of any known polymorph of sulfur (Table 15.6). 

Different modifications of a compound are frequently designated by lower case Greek letters a, j3,..., e.g. a-sulfur, j3-sulfur, or by roman numerals, e.g. tin-I, tin-II etc. Polymorphic forms of minerals have in many cases been given trivial names, like a-quartz, P-quartz, tridymite, cristobalite, coesite, keatite, and stishovite for Si02 forms. 

Figure 1.6 Heat capacity of rhombic and monoclinic sulfur [4,5] and the derived entropy of transition between the two polymorphs.

The third law of thermodynamics can be verified experimentally. The stable rhombic low-temperature modification of sulfur transforms to monoclinic sulfur at 368.5 K (p = 1 bar). At that temperature, Ttrs, the two polymorphs are in equilibrium and the standard molar Gibbs energies of the two modifications are equal. We therefore have... 

Concentration standards, 75 750-751 Concentrators, sulfuric acid, 23 787 Concentric annular reactors, 23 544 Concentric cylinder viscometer, 27 733 Concentric hemispherical analyzer (CHA), 24 103-104, 105 energy resolution of, 24 106 Conching, milk chocolate, 6 363-364 Concomitant polymorphism, 8 69 CONCORD program, 6 10 76 752 Concrete(s)... 

The similarly blue and equally polymorphous metal-free phthalocyanine existing in five different crystal modifications (a, (3, y, k, t) is chemically somewhat less stable than its copper complex [26] it decomposes slowly in a sulfuric acid solution. On the other hand, it can be chlorinated to afford metal-free Phthalocyanine Green. 

Three of the polymorphs of cyclooctasulfur are firmly established orthorhombic a-sulfur, monoclinic j8-sulfur, and monoclinic y-sulfur. Their structure, along different axes, is shown in Fig. 6. Over a dozen of... 

Yellow to orange crystal occurs as two polymorphs, hexagonal alpha form and cubic beta form exhibits stable wurtzite structure at lower temperature, and zinc blende type structure at higher temperatures the beta form converts to alpha form when heated at 750°C in sulfur atmosphere sublimes at 980°C practically insoluble in water (1.3 mg/L at 20°C) Ksp 3.6x10-29 dissolves in dilute mineral acids on heating or concentrated acids at ordinary temperatures (decomposes with liberation of H2S). 

Sulfur trioxide is prepared in the laboratory by heating fuming sulfuric acid, condensing its vapors, and collecting in a cool receiver. When vapors are condensed below 27°C in the presence of trace moisture, aU three polymorphic phases of SO3 are produced. They can be separated by fractional distillation. Condensation of the vapors above 27°C forms the hquid variety of gamma-sulfur trioxide. 

The methyl transferases (MTs) catalyze the methyl conjugation of a number of small molecules, such as drugs, hormones, and neurotransmitters, but they are also responsible for the methylation of such macromolecules as proteins, RNA, and DNA. A representative reaction of this type is shown in Figure 4.1. Most of the MTs use S-adenosyl-L-methionine (SAM) as the methyl donor, and this compound is now being used as a dietary supplement for the treatment of various conditions. Methylations typically occur at oxygen, nitrogen, or sulfur atoms on a molecule. For example, catechol-O-methyltransferase (COMT) is responsible for the biotransformation of catecholamine neurotransmitters such as dopamine and norepinephrine. A-methylation is a well established pathway for the metabolism of neurotransmitters, such as conversion of norepinephrine to epinephrine and methylation of nicotinamide and histamine. Possibly the most clinically relevant example of MT activity involves 5-methylation by the enzyme thiopurine me thy Itransf erase (TPMT). Patients who are low or lacking in TPMT (i.e., are polymorphic) are at... 

PbO PbO exists in two polymorphic forms - tetragonal a-PbO and orthorhombic yS-PbO they are photosensitive substances. Basic electrochemical properties of PbO have been studied in acidic [105, 111], neutral [165,] and alkaline [153] media. In acidic solutions, the /I-PbO formation is followed by transformation to a- PbO [166, 167]. In sulfuric acidic... 

Systematic studies of the structural properties of TaS2 by Jellinek10 revealed the presence of several polymorphic forms of the compound as a function of temperature. Within a layer, the tantalum atom sits in the holes formed between two layers of sulfur atoms in the S—Ta—S sandwich. The coordination of the tantalum is trigonal prismatic or octahedral, depending on whether the two sulfur layers lie one on top of the other or are rotated by 60°. Thus the various poly-... 

POLYMORPHISM. 1, A phenomenon in which a substance exhibits different forms. Dimorphic substances appear in two solid forms, whereas trimorphic exist in three, as sulfur., carbon, tin, silver iodide, and calcium carbonate. Polymorphism is usually restricted to the solid state, Polymorphs yield identical solutions and vapors (if vaporizable). The relation between them has been termed physical isomerism. See Allotropes under Chemical Elements, See also Mineralogy,... 

Barium Sulfate (Blanc fixe, Actybaryte, Bakontal, Baridol, Citobaryium, Neobar or Unibaryt). BaS04 mw 233.43 fine, heavy, odorless powdr or polymorphous cryst mp 1600° (decompn) d 4.25—4.58g/cc. Sol in hot coned sulfuric acid ... 

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