Rhombohedral sulfur

E. Beckmann showed that rhombohedral sulfur was S3 by cryoficopy in molten iodine. 

P. A. G. O Hare, S. Susman, K. J. Volin, S. C. Rowland. Combustion Calorimetry of Rhombohedral Sulfur in Fluorine A Question of Impurities. The Standard Molar Enthalpies of Formation ofSF (g) and SCpf (aq) at the Temperature of298.15 K. J. Chem. Thermodynamics 1992, 24, 1009-1017. 

Sulfur can form rings and chains of various shapes and sizes. In addition to the crown-shaped cyclo-Sg, nine other types of rings can be prepared the first of these, cyclohexasulfur, also called Engel s or Aten s sulfur, or rhombohedral sulfur, Sp, was first described by Engel in 1891 (28). The other eight types of rings were all discovered during the last 10 years by Schmidt and his co-workers (87,88) notably Wilhelm (125). 

Frasch develops process for recovery of sulfur Rhombohedral sulfur established as Sg... 

Rhombohedral sulfur Aten sulfur Engel sulfur e-Sulfur p-Sulfur Rhombohedral (p) sulfur Crystalline cyclo-Se Space group R3... 

Rhombohedral sulfur (the p-form) comprises Sg rings and is obtained by the ring closure reaction 15.17. It decomposes in light to Sg and S12. 

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... 

At least five high-pressure allotropes of sulfur have been observed by Raman spectroscopy up to about 40 GPa the spectra of which differ significantly from those of a-Sg at high pressures photo-induced amorphous sulfur (a-S) [57, 58, 109, 119, 184-186], photo-induced sulfur (p-S) [57, 58, 109, 119, 184, 186-191], rhombohedral Se [58, 109, 137, 184, 186, 188, 191], high-pressure low-temperature sulfur (hplt-S) [137, 184, 192], and polymeric sulfur (S ) [58, 109, 119, 193]. The Raman spectra of two of these d-lotropes, a-S and S, were discussed in the preceding section. The Raman spectra of p-S and hplt-S have only been reported for samples at high-pressure conditions. The structure of both allotropes are imknown. The Raman spectrum of Se at STP conditions is discussed below. 

Rhombohedral Se was found as a high-pressure allotrope of sulfur above 9-10 GPa by several groups [58, 137, 150, 184, 186, 188, 191]. The pressure dependence of frequencies [137, 150, 184] as well as the kinetics of the transition from p-S to Ss [186] have been investigated systematically by Raman spectroscopy. The pressure dependent frequency shifts of chemically prepared Ss and of high-pressure Ss have been found to be identical [137, 150]. 

The substance can be crystallized from toluene or CS2. It forms orange-red rhombohedral crystals with a density of 2.209 gm/cm , the highest density of any known sulfur allotrope. Eighteen Sg molecules occupy a unit cell with the space group SS-C i. The lattice constants are... 

Colorless crystalline solid rhombohedral structure decomposes at 73.5°C decomposes in water soluble in sulfuric acid. 

The most common valence states of arsenic are —3, 0, +3, and +5 (Shih, 2005), 86. The —3 valence state forms through the addition of three more electrons to fill the 4p orbital. In the most common form of elemental arsenic (As(0)), which is the rhombohedral or gray form, each arsenic atom equally shares its 4p valence electrons with three neighboring arsenic atoms in a trigonal pyramid structure ((Klein, 2002), 336-337 Figure 2.1). The rhombohedral structure produces two sets of distances between closest arsenic atoms, which are 2.51 and 3.15 A (Baur and Onishi, 1978), 33-A-2. The +3 valence state results when the three electrons in the 4p orbital become more attracted to bonded nonmetals, which under natural conditions are usually sulfur or oxygen. When the electrons in both the 4s and 4p orbitals tend to be associated more with bonded nonmetals (such as oxygen or sulfur), the arsenic atom has a +5 valence state. 

Another unique structure is found in the rhombohedral room-temperature modifications of NiS (millerite) and NiSe, shown in Fig. 50. The anions form infinite trigonal prisms. The nickel atoms are located slighty above the square faces of these prisms. The coordination polyhdra of the nickel atoms are distorted square pyramids of anions. However, in millerite the nickel atoms of the other two faces of the sulfur prisms are as close as 2.53 A which roughly corresponds to half bonds. There is, therefore, little doubt that NiS is metallic which would be in agreement with the observed Pauli paramagnetism (55). 

A form of crystalline sulfur with rhombohedral symmetry can be made by extracting an acidified solution of sodium thiosulfate with chloroform and evaporating the chloroform solution. These crystals, which are orange in color, consist of molecules they are unstable, nd change into long chains and then into orthorhombic sulfur (Sg) in a few hours. 

Class I. ELEMENTS. A. Metals. Cubic copper, silver, gold, iron, platinum, iridium. - Tetragonal tin. - Rhombohedral and Hexagonal arsenic, antimony, bismuth, tellurium, (Os, Ir). - B. Metalloids. Cubic diamond. - Hexagonal graphite. - Orthorhombic sulfur, iodine. - Monoclinic sulfur, selenium. - Class II. SULFIDES. - Class HI. HALIDES. -Class IV. OXIDES, divided into SIMPLE OXIDES and COMPLEX OXIDES, such as CARBONATES, PHOSPHATES, SILICATES, BORATES and SULFATES. 

As listed in Table 2 and shown in Fig. 1, each of the Ss molecules has 18 short intermolecular contacts in the range 350-353 pm (at 300 K). This fact, in combination with the compact molecular structure, accounts for the high density of rhombohedral 85 (for comparison see the structure data of orthorhombic sulfur. Table 6). On the other hand, the compact molecular structure is responsible for a certain strain in the bond geometry which is expressed by a relatively large deviation of the torsion angle from an unstrained value of about 90°, therefore, making the molecule unstable [66]. 

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