Big Chemical Encyclopedia

Chemical substances, components, reactions, process design ...

Articles Figures Tables About

Periodic crystalline materials

One of the attractions of supramolecular chemistry is the extraordinary potential for synthesis of new materials that can be achieved much more rapidly and more effectively than with conventional covalent means. For supramolecular synthesis to advance, it is obviously important to characterize, classify, and analyze structural patterns, space group frequencies, and symmetry operators [118], However, at the same time we also need to bring together this information with the explicit aim of improving and developing supramolecular synthesis - the deliberate combination of different discrete molecular building blocks within periodic crystalline materials. [Pg.225]

There exist several different approaehes for meeting these challenges. Experimental information and/or ehemieal intuition ean be used in identifying reasonable structures that can be studied with aeeurate methods. Thereby, information on the symmetry properties of the system of interest can be of enormous help it makes, e.g., theoretieal studies on infinite, periodic, crystalline materials possible. Alternatively, for systems for whieh little or no information on the structure is available, theoretieal studies with simpler, parameterized methods can be very helpful. These studies may eventually be... [Pg.511]

An approximant is a periodic crystalline material that is closely related to a QC, both in chemical composition and in atomic structure. Often, an approximant contains the same clusters as those embedded in the QC of related composition [3]. [Pg.349]

Since the recognition in 1936 of the wave nature of neutrons and the subsequent demonstration of the diffraction of neutrons by a crystalline material, the development of neutron diffraction as a useful analytical tool has been inevitable. The initial growth period of this field was slow due to the unavailability of neutron sources (nuclear reactors) and the low neutron flux available at existing reactors. Within the last decade, however, increases in the number and type of neutron sources, increased flux, and improved detection schemes have placed this technique firmly in the mainstream of materials analysis. [Pg.648]

Acetylene is passed for 1 hr through a mixture consisting of 0.5 g (72 mg-atoms) of lithium in 100 ml of ethylene-diamine. A solution prepared from 1 g (3.5 mmoles) of rac-3-methoxy-18-methylestra-l,3,5(10)-trien-I7-one and 30 ml of tetrahydrofuran is then added at room temperature with stirring over a period of 30 min. After an additional 2 hr during which time acetylene is passed through the solution the mixture is neutralized with 5 g of ammonium chloride, diluted with 50 ml water, and extracted with ether. The ether extracts are washed successively with 10% sulfuric acid, saturated sodium hydrogen carbonate and water. The extract is dried over sodium sulfate and concentrated to yield a solid crystalline material, which on recrystallization from methanol affords 0.95 g (87%) of rac-3-methoxy-18-methyl-17a-ethynyl-estra-l,3,5(10)-trien-17jB-ol as colorless needles mp 161°. [Pg.73]

A difiiculty with this mechanism is the small nucleation rate predicted (1). Surfaces of a crystal with low vapor pressure have very few clusters and two-dimensional nucleation is almost impossible. Indeed, dislocation-free crystals can often remain in a metastable equilibrium with a supersaturated vapor for long periods of time. Nucleation can be induced by resorting to a vapor with a very large supersaturation, but this often has undesirable side effects. Instabilities in the interface shape result in a degradation of the quality and uniformity of crystalline material. [Pg.219]

Commonly an amorphous solid may have solubility in both aqueous and DMSO that is over 100 hmes higher than any of the crystalline forms. An amorphous solid can be inadvertently created from a crystalline material, e.g. by heating a crystalline hydrate or solvate for a short time period below its melhng point in a drying pistol. [Pg.271]

Upon standing, the mother liquor deposited more allitol—some 3 g. after a period of two months. No crystalline material was obtained from the ethereal solution. We thus obtained from 100 g. of divinylglycol, 11 g. of D,L-mannitol and 15 g. of allitol. [Pg.129]

The technique generally used to minimize the degree of change in crystallinity of the milled product is to eliminate the water or other solvents from the product, usually by packaging the material within a suitable barrier (for example, aluminum foil laminate). Other techniques include the production of a 100% crystalline material, which may eliminate the effects of moisture. This technique, however, may require a secondary production stage of armealing or a period to allow the product to equilibrate under controlled storage conditions. [Pg.102]

The first report in the literature of the isolation of a substance recognizable as a cyclodextrin was that of Villiers which appeared in 1891. From digests of Bacillus amylobacter on potato starch, Villiers obtained a small amount (3 g per 1000 g of starch) of a crystalline material, which he named cellulosine because of its resemblance in some respects to cellulose. The foundations of cyclodextrin chemistry were laid down, however, in the period 1903-1911 by Schardinger, and, in fact, some of the older literature frequently refers to the cyclodextrins as Schardinger dextrins. [Pg.211]

When X-rays illuminate a crystalline material, the atoms in the crystal act as scattering centers. Because of the periodic nature of crystals, the scatterers can be considered to be associated with periodically spaced parallel planes a distance d apart. For certain angles of incidence to these planes the X-rays are scattered coherently and in phase. The coherent scattering is known as X-ray diffraction and the geometric condition required for diffraction, the Bragg equation, is given by... [Pg.466]

In situations where V(x9y9z) is not periodic, as for instance in the impurity band of doped semiconductors and in non-crystalline materials, it is still true that if N(E ) vanishes then the material is an insulator at zero temperature, but the converse is not true. This is because a finite value of N(E )9 still within the context... [Pg.19]


See other pages where Periodic crystalline materials is mentioned: [Pg.284]    [Pg.161]    [Pg.285]    [Pg.333]    [Pg.242]    [Pg.110]    [Pg.194]    [Pg.472]    [Pg.93]    [Pg.134]    [Pg.112]    [Pg.809]    [Pg.226]    [Pg.564]    [Pg.76]    [Pg.458]    [Pg.294]    [Pg.146]    [Pg.78]    [Pg.9]    [Pg.304]    [Pg.110]    [Pg.64]    [Pg.27]    [Pg.29]    [Pg.14]    [Pg.346]    [Pg.134]    [Pg.139]    [Pg.245]    [Pg.96]    [Pg.285]    [Pg.333]    [Pg.294]   
See also in sourсe #XX -- [ Pg.225 ]




SEARCH



Crystalline periodicity

© 2024 chempedia.info