Orthorhombic

Although several early attempts had been made to analyze the structure of this allotrope, the first correct description of the crystalline as well as of the molecular structure was given by Warren and Burwell in 1935 [82], which was later improved by Abrahams in 1955 [83]. The most recent examination of the structure of a-Sg by Rettig and Trotter in 1987 confirmed the results of the earlier studies with a somewhat higher precision [80]. The history of the attempts to characterize the structure of a-Sg up to the 1970s has been discussed by Donohue [8]. 

In a-Sg the molecules crystalhze in the orthorhombic space group Fddd-Dih. The octamers are arranged in two layers each perpendicular to the crystal c axis forming a so called crankshaft structure (Fig. 5). The primitive cell contains four molecules on sites of C2 symmetry. Four non- 

The molecular distortion from a perfect D4d symmetry was discussed by Pawley, Rinaldi and Kurittu by means of a constrained refinement of experimental data and by theoretical calculations which made use of intermolecular and intramolecular force fields available at that time [84, 85]. 

Results of recently performed molecular dynamics calculations using isotropic and anisotropic Ss model molecules for simulation of crystalline phases suggested the existence of a metastable monoclinic phase below 200 K 

This allotrope is usually obtained by heating of powdered a-Sa to about 369 K. In single crystals of a-Ss the transformation is kinetically hindered, for example due to the absence of impurities introduced by grain boundaries. Another way to obtain pSs is by slow cooling of molten sulfur, or by crystallization from organic solvents. 

Dielectric properties Thermal expansion coefficient Pyroelectric properties 

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For a free energy of fonnation, the preferred standard state of the element should be the thennodynamically stable (lowest chemical potential) fonn of it e.g. at room temperature, graphite for carbon, the orthorhombic crystal for sulfiir. 

Abstract. This paper presents results from quantum molecular dynamics Simula tions applied to catalytic reactions, focusing on ethylene polymerization by metallocene catalysts. The entire reaction path could be monitored, showing the full molecular dynamics of the reaction. Detailed information on, e.g., the importance of the so-called agostic interaction could be obtained. Also presented are results of static simulations of the Car-Parrinello type, applied to orthorhombic crystalline polyethylene. These simulations for the first time led to a first principles value for the ultimate Young s modulus of a synthetic polymer with demonstrated basis set convergence, taking into account the full three-dimensional structure of the crystal. 

Fig. 4. Structure of orthorhombic polyethylene unit cell projected on the ab plane. The right-hand pictures define the setting angle .

Regarding mechanical properties of polymers, the efficiency of the Car-Parrinello approach has enabled us to evaluate the ultimate Young s modulus of orthorhombic polyethylene, and demonstrate basis set convergence for that property. 

Orthorhombic (or rhombic) Three unequal axes mutually perpendicular Three mutually perpendicular twofold axes, or two planes intersecting in a twofold axis a b c a = /3 = 7 = 90°... 

The many commercially attractive properties of acetal resins are due in large part to the inherent high crystallinity of the base polymers. Values reported for percentage crystallinity (x ray, density) range from 60 to 77%. The lower values are typical of copolymer. Poly oxymethylene most commonly crystallizes in a hexagonal unit cell (9) with the polymer chains in a 9/5 helix (10,11). An orthorhombic unit cell has also been reported (9). The oxyethylene units in copolymers of trioxane and ethylene oxide can be incorporated in the crystal lattice (12). The nominal value of the melting point of homopolymer is 175°C, that of the copolymer is 165°C. Other thermal properties, which depend substantially on the crystallization or melting of the polymer, are Hsted in Table 1. See also reference 13. 

Acetamide [60-35-5] C2H NO, mol wt 59.07, is a white, odorless, hygroscopic soHd derived from acetic acid and ammonia. The stable crystalline habit is trigonal the metastable is orthorhombic. The melt is a solvent for organic substances it is used ia electrochemistry and organic synthesis. Pure acetamide has a bitter taste. Unknown impurities, possibly derived from acetonitrile, cause its mousy odor (1). It is found ia coal mine waste dumps (2). 

AmBr white to pale yellow orthorhombic TbCl3 0.4064 1.266 0.914 6.85... 

CmBr pale yellow-green orthorhombic TbCl3 0.4041 1.270 0.913 6.85... 

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