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Monoclinic form

Tripotassium hexakiscyanoferrate [13746-66-2] K2[Fe(CN)g], forms anhydrous red crystals. The crystalline material is dimorphic both orthorhombic and monoclinic forms are known. The compound is obtained by chemical or electrolytic oxidation of hexacyanoferrate(4—). K2[Fe(CN)g] is soluble in water and acetone, but insoluble in alcohol. It is used in the manufacture of pigments, photographic papers, leather (qv), and textiles and is used as a catalyst in oxidation and polymerisation reactions. [Pg.435]

Anhydrous Oxalic Acid. The anhydrous form of oxaUc acid is odorless and colorless. It exists in two crystal forms, ie, the rhombic or a-form and the monoclinic or P-form (3). The rhombic crystal is thermodynamically stable at room temperature, but the monoclinic form is metastable or slightly stable. The main difference between the rhombic and monoclinic forms exists in the melting points which are 189.5 and 182°C, respectively (Table 1)-... [Pg.456]

Sulfur crystallizes in at least two distinct systems the rhombic and the monoclinic forms. Rhombic sulfur, Sa, is stable at atmospheric pressures up to 95.5°C, at which transition to monoclinic sulfur, SP, takes place. Monoclinic sulfur is then stable up to its natural melting point of 114.5°C. The basic molecular unit of both of these crystalline forms of sulfur is the octatomic sulfur ring Other forms of sohd sulfur include hexatomic sulfur as well as... [Pg.115]

Zirconia prepared by the thermal decomposition of zirconium salts is often metastable tetragonal, or metastable cubic, and reverts to the stable monoclinic form upon heating to 800°C. These metastable forms apparently occur because of the presence of other ions during the hydrolysis of the zirconium their stabiUty has been ascribed both to crystaUite size and surface energy (152—153) as well as strain energy and the formation of domains (154). [Pg.434]

The hydroxides as precipitated are amorphous, but if they are refluxed ia a neutral or slightly acidic solution they convert to a mixture of cubic and monoclinic hydrous zirconia crystaUites on continued refluxing, only the monoclinic form persists (196). If the refluxing is conducted in an alkaline solution, metastable cubic zirconia is formed (197). [Pg.437]

The structure of N2O4 in the gas phase is planar (D2h) with a remarkably long N -N bond, and these features persist in both the monoclinic crystalline form near the mp and the more stable low-temperaturc cubic form. Data for the monoclinic form are in the lower diagram together with those for the isoelectronie species B2F4 and... [Pg.455]

The second choice is a simpler solution. According to Sarko and Muggli,66 all 39 observed reflections in the Valonia X-ray pattern are indexable by a two-chain triclinic unit cell with a = 9.41, b =8.15 and c = 10.34 A, a = 90°, 3 = 57.5°, and y = 96.2°. Ramie cellulose, on the other hand, is completely consistent with the two-chain monoclinic unit cell. Also, there are significant differences between their high-resolution solid-state l3C NMR spectra, indicating that Valonia and ramie celluloses, the two most crystalline forms, reflect two distinct families of biosynthesis. On this basis, the Valonia triclinic and the ramie monoclinic forms are classified69 as Ia and Ip, respectively. It has been shown from a systematic analysis of the NMR spectra by these authors, and from electron-dif-... [Pg.330]

Because of the conversion of orthorhombic sulfur to monoclinic form, the above values of melting points are difficult to observe, as the resulting allotropic mixture melts at only 115 C. Amorphous or plastic sulfur can be produced through the rapid cooling of molten sulfur. X-ray crystallographic studies show that the amorphous form may have a helical structure with eight atoms per turn. This form is metastable at room temperature and gradually reverts back to crystalline within hours to days but this conversion can be rapidly catalyzed. [Pg.8]

Two simple supports have been used, a transition y (5) alumina (around 120 m gr ) and a tetragonal Z1O2 (70 m g- ) with a low amount of the monoclinic form... [Pg.346]

CflHuBrNjOij 5-Bromo-2 -deoxyuridine (monoclinic form) BROXURIO 31 369... [Pg.406]

Methylsull anyT I //-tetrazole was found to crystallize in a monoclinic form, and could be sublimed into an orthorhombic form, with the structures differing in the relative polarity of the molecular layers in the two forms [47]. /) -1 o d o a ce t o p h e n one was found to crystallize in two polymorphs that both contained C—H-re points of contact, but where the contacts were shorter in one form than in the other [48]. A second monoclinic modification of the mixed salt benzimidazolium 3-carboxyphe-noxyacetate 3-carboxyphenoxyacetic acid was reported, where the acid hydrogen atom and the two monoanions comprised a carboxylate monoanion/neutral molecule in which the acid proton was disordered between the two anionic units [49]. [Pg.269]

The structure of a second polymorph of 4,5-diphenyl- lH-imidazole has been discussed, with the new form exhibiting significantly different phenyl/imidazole dihedral angles and mode of crystal packing relative to the known form [53], A new triclinic polymorph of 1,4-dibenzoyl-butane was found, differing from the monoclinic form in the torsional angles of the central chain [54], Two polymorphs of diphenyl-(4-pyridyl)methyl methacrylate have been found, where the molecules in the two forms contain weak C—H— n and C—H O/N contacts that lead to the existence of different conformations [55]. [Pg.270]

A new photoactive monoclinic polymorph of 6-(2, 4 -dinitrobenzyl)-2,2 -bipyri-dine was obtained from an acetone/methanol solution, and the structure compared to the previously known photoactive orthorhombic and photoinactive monoclinic forms [94]. Correlation of these structures with those of related nitrobenzylpyridines was used to understand the relationships existing between structure and photochro-mism. The comparison of the reaction cavities around the reactive pyridyl-benzyl-nitro fragment indicated that photochromic activity required rotational freedom of the ortho-nitro group in the crystal and its accessibility from the proton-donor and proton-acceptor sites. [Pg.276]

There are two basic crystallisation forms for sulphur - monoclinic and rhombic. Rhombic is the most stable form, at least up to 96 °C the other types revert to this stable form at a rate dependant upon temperature. When sulphur solidifies from the molten state (melting point 114 °C) the crystalline form which occurs is monoclinic (needle-like crystal structure). Below 96 °C, the monoclinic form becomes metastable and changes into the rhombic form. [Pg.161]

Figure 3. Schematic illustration of the disposition of chains made up of cornersharing SiOk tetrahedra in the triclinic and monoclinic forms of wollastonite, (The Ca2+ ions have been omitted for clarity) (top), and (hOl) and (hll) x-ray diffraction patterns that signify that the specimen in question is disordered and made up of intergrowths of the triclinic and monoclinic forms (bottom). Figure 3. Schematic illustration of the disposition of chains made up of cornersharing SiOk tetrahedra in the triclinic and monoclinic forms of wollastonite, (The Ca2+ ions have been omitted for clarity) (top), and (hOl) and (hll) x-ray diffraction patterns that signify that the specimen in question is disordered and made up of intergrowths of the triclinic and monoclinic forms (bottom).
Corrected for molecular libration. Monoclinic form. cLow-temperature form there are two molecules per asymmetric unit Molecule A. Molecule B. f There are two molecules per asymmetric unit. one is neutral and die other a zwitterion. ... [Pg.94]

Tliree values are reported two for molecules in the monoclinic form, and one for the orthorhombic one x indicates C-N bond y indicates N—N bond. [Pg.96]

Fig.29 Interlayer structure of [Ca2Al(0H)6]Cl-2H20 the high-temperature rhombohe-dral form (represented by a projection along the Ch axis is shown on the left and the low-temperatiue monoclinic form (represented by a projection on the (Um, bm) plane is shown on the right. Reprinted with permission from [112]. Copyright Elsevier Science Ltd... Fig.29 Interlayer structure of [Ca2Al(0H)6]Cl-2H20 the high-temperature rhombohe-dral form (represented by a projection along the Ch axis is shown on the left and the low-temperatiue monoclinic form (represented by a projection on the (Um, bm) plane is shown on the right. Reprinted with permission from [112]. Copyright Elsevier Science Ltd...

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See also in sourсe #XX -- [ Pg.447 ]




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