Crystalline stucture

Other Crystalline Forms.—In addition to the foregoing crystalline modifications of sulphur, at least two others have been observed. When using boiling sulphur as the heating agent in the outer jacket of a Victor Meyer vapour density apparatus, Friedel,2 in 1879, noticed the formation of very unstable triclinic crystals inside the upper part of the jacket these crystals rapidly underwent change into ordinary octahedral sulphur, Various crystalline stuctures may be observed when sulphur condenses in droplets on a glass jdate and solidifies.3 Such crystals axe also unstable. 

MFI and FER containing composites and fully crystalline materials of both stuctural types could be realized if the propylamine route was achieved. By varying the SiCVAbOj-ratio in the starting reaction mixture the crystallization could be directed to the FER or MFI products. 

In 1994, the ab initio X-ray crystal stucture determination from powder diffraction data of [Cu(pz)] and [Ag(pz)] were reported (13). During these studies it was ascertained that copper(I) and silver(I) pyrazolates, depending on the synthetic method used, each appear in two distinct crystalline phases. The a-[Cu(pz)] , 2a, and /3-[Cu(pz)] , 2b, consist of infinite chains of linearly coordinated copper atoms, bridged by exo-bidentate pyrazolate anions (Fig. 2). The two phases differ mainly in the interchain Cu- -Cu contacts, which are 3.34 A in a-[Cu(pz)] and 2.97 A in 3-[Cu(pz)] . 

We conclude this survey with extended systems, where the assembled structures show the translational symmetry characteristic of the crystalline state. The packing of chiral units into a crystal lattice will inevitably involve some type of diastereo-selectivity, either homochiral or heterochiral, although this is fiequently not discussed in crystal stucture reports. If the associations are all homochiral, then an enantiomerically pure crystal will be obtained, and a solution of the racemate will yield a racemic mixture (see section 3). If, on the other hand, heterochiral association (often related by a centre of inversion or glide plane) is favoured, a racemic compound will crystallize. The double helicate [ 02(51)2] " crystallizes with a homochiral association of complexes along the helieal axis (Figure 38). The homochiral columns are then arranged in pseudohexagonal arrays with the ehirality... 

A lone exception has been reported describing the isolation of two crystalline forms of p-hexanoylcalix[4]arene which, on the basis of CP-MAS NMR spectral measurements, are assigned cone (one C=0 resonance) and partial cone (3 C=0 resonances) stuctures. 

Values of glass transition temperature (Tg) and crystalline melting temperature (T ) for the various polybutadiene structures are given in Table 3. CrystaUinity arising from chain stucture regularity can occur... 

These are the fluctuations observed in a structure, when a phase transition is approached, and which can be considered as precursors of the transformation. In general their local symmetry is closely related to that of the stucture to come. For instance, in the case of the solid/Iquid transition for which this term was proposed first , they are local perturbations of the crystalline order when the transition to the liquid is approached. Similar phenomena have indeed been observed in the case of the lame liar/cubic transformation in Cj2E0g/H20. They manifest themselves, on the X-ray pattern of the... 

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