Orthorhombic crystalline form

Black phosphorus is the third major adotropic form of phosphorus. It occurs in two forms, one is an amorphous modification having a laminar structure similar to graphite and the other is an orthorhombic crystalline form. The density of black phosphorus may vary between 2.20 to 2.69 g/cm. Black phosphorus is obtained from white phosphorus by heating the latter at 220°C under an extremely high pressure of about 10,000 atm. 

The principal features of elemental sulfur in the displayed T, P range are the usual liquid and vapor phases and two solid forms, a-sulfur ( red sulfur, of orthorhombic crystalline form) and /3-sulfur ( yellow sulfur, monoclinic needle-like crystals), both of which are available as common stockroom species. The stable phase ranges for each elemental form are shown by the solid lines in Fig. 7.5. The liquid-vapor coexistence line terminates in a critical point at 1041°C, and will not be discussed further. 

The solid-state structure for stannocene has not been determined the crystal structure for the orthorhombic crystalline form of plumbocene is shown in Fig. 2 (17), Plumbocene is associated in the solid state, with each lead atom coordinated to three cyclopentadienyl rings, one of them terminal and two of them bridging to other lead atoms. 

Spin-Lattice Relaxation. In order to determine whether each resonance line comprises a single component, we first measured the spin-lattice relaxation time Tic by the pulse sequence developed by Torchia [53] or by the standard saturation-recovery pulse sequence. The Tic values thus obtained were 2560,263 and 1.7 s for resonance line I and 0.37 s for line II. As reported by several investigators, the line at 33 ppm is associated with three different Tic values [ 17,54,55]. This means that this line is contributed to by three components with different molecular mobilities. However, since each component was represented by a single Lorentzian line shape at 33 ppm, they are all assignable to methylene groups in the orthorhombic crystalline form or in the trans-trans conformation. The component with a Tic of s can be assigned to methylene groups with a some-... 

Cooper SM. Orthorhombic crystalline form of fluticasone propionate and pharmaceutical composition thereof US patent 6,406,718, 2002. 

Black Phosphorus. Polymorphic. Orthorhombic crystalline form stable in air resembles graphite in texture produced from the white modification under high pressures Bridgman, J. Am. Chem. Soc. 36, 1344 (1914) Jacobs, J. Chem. Phys. 5, 945 (1937) Krebe, Inorg. Syn. 7, 60 (1963), d 2.691. Does not catch fire spontaneously. Insol in organic solvents. Amorphous form prepd at lower pressures Ja-cobe, loc. cit At higher pressure the orthorhombic form undergoes reversible transition to a rhombohedral structure, d 3.56, and a cnbic structure, d 3.83 Jamieson, Science 139, 1291 (1963). 

Since sulfur, without additives, has limited strength and becomes brittle as it reverts to the orthorhombic crystalline form, the investigators expected major problems with sulfur impregnation. For example, sawing and nailing operations might lead to fissures and cracks which would be... 

There are also two orthorhombic crystalline forms which contain highly polymerised arrangements (P205) . The O form or Form IP, density 3.05 g/cc, mp = 580°C, has a sheet structure built from rings of six PO4 tetrahedra (4.66b). The O form (Form III), density 2.72 g/cc, mp = 562°C, is built from puckered rings of 10 PO4 tetrahedra which are linked laterally to form a three-dimensional structure. Another solid form of unknown structure exists at high pressures. The vapour from these forms is stable up to at least 1400°C, and at high temperatures the existence of dimeric species (4.66a) is possible. 

According to X-ray studies, in the most frequent orthorhombic crystalline form of polyethylene, the chains assume the planar zigzag ttt conformational structure [2]. The unit cell contains two polyethylene chains, and this is also manifested in IR spectra, where many bands exhibit characteristic doublets caused by vibrational dipolar interactions of polymer chains [3]. Several other crystalline modifications of polyethylene... 

In general, the chemistry of inorganic lead compounds is similar to that of the alkaline-earth elements. Thus the carbonate, nitrate, and sulfate of lead are isomorphous with the corresponding compounds of calcium, barium, and strontium. In addition, many inorganic lead compounds possess two or more crystalline forms having different properties. For example, the oxides and the sulfide of bivalent lead are frequendy colored as a result of their state of crystallisation. Pure, tetragonal a-PbO is red pure, orthorhombic P PbO is yeUow and crystals of lead sulfide, PbS, have a black, metallic luster. 

Physical Properties. LLDPE is a sernicrystaUine plastic whose chains contain long blocks of ethylene units that crystallize in the same fashion as paraffin waxes or HDPE. The degree of LLDPE crystallinity depends primarily on the a-olefin content in the copolymer (the branching degree of a resin) and is usually below 40—45%. The principal crystalline form of LLDPE is orthorhombic (the same as in HDPE) the cell parameters of nonbranched PE are a = 0.740 nm, b = 0.493 nm, and c (the direction of polymer chains) = 0.2534 nm. Introduction of branching into PE molecules expands the cell slightly thus a increases to 0.77 nm and b to around 0.50 nm. 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

Besides the amorphous red P which is a commercial product, there are perhaps five other crystalline or poorly crystalline red modifications that are not produced commercially. These include a triclinic, an orthorhombic, a high temperature form, and two poorly crystalline forms. Red phosphoms varieties are rather stable in air and are of lower reactivity than white phosphoms. 

A specific polymorph may be absolutely essential for a crystalline product, for example, one polymorph may have a more desirable color or greater hardness or disperse in water more easily than another polymorph. Often, one polymorphic form is more stable than another (for example, at 80°C the orthorhombic I form of ammonium nitrate is more stable than the trigonal form) at conditions to which a product is exposed. An interesting approach to... 

For instance, in the three crystalline forms (a, 3, y) of i-PP the chains are always in the conformation of threefold helix (s(3/l)l symmetry) but are packed in different ways in monoclinic [24], hexagonal [25], and orthorhombic [26] unit cells, respectively. The X-ray diffraction spectra of unoriented samples in the crystalline forms a, P, y are reported in Fig. 6. 

The small difference in energy between S4 and Ci forms caused speculations as to whether a second crystalline form might exist which has S4 symmetry. These assumptions were fed by the fact that an X-ray powder diffractogram revealed another orthorhombic lattice with half of the volume. This polymorphic form emerged when cooling below the... 

Silvery-white metal exhibits three crystalline modifications an orthorhombic alpha form, stable at ordinary temperatures and density 20.45 g/cm the alpha-form transforms to a tetragonal beta allotrope of density 19.36 g/cm when heated at 280°C the beta form converts to a body-centered cubic crystaUine gamma modification at 577°C, having a density 18.0 g/cm . 

Alpha cyclooctasulfur or the alpha-sulfur is yellow orthorhombic crystalline solid refractive index 1.957 density 2.07 g/cm at 20°C stable at ordinary temperatures converts to monoclinic beta form at 94.5°C. 

The degree of crystallinity may be calculated from the density of the polymer if the density is known for the amorphous and crystalline states. Some crystallizable polymers are polymorphic, i.e., they may exist in more than one crystalline form. An unstable crystalline form may change to a more stable form, and crystalline forms may change under stress. For example, hdpe changes from an orthorhombic crystalline polymer to a monoclinic form when subjected to compressive forces. 

It is seen that the 1-3 linkage requires a significantly larger distance than the other linkages and hence is much less probable for near-neighbor interactions. Therefore, we predict the dimeric isomers and yields shown in Table IX. Because of the arrangement of the carbon atoms in the orthorhombic and monoclinic crystals, the same dimer distribution is also predicted for them, and no change in the dimeric isomer distribution would be expected for any of the reported crystalline forms. 

---