Related Structures

The strontium titanium sulfides, SrvTiSwhich exist between the x values of 1.05 and 1.22, contain interpenetrating rods of structure, both of which are modulated. In idealized Sr,TiS i, face-sharing TiS6 octahedra (Fig. 4.38a) form columns with a repeat 

The real structures of these phases are more complex. The coordination of the Ti atoms is always six, but the coordination polyhedron of sulfur atoms around the metal atoms is in turn modulated by the modulations of the Sr chains. The result of this is that some of the TiS, polyhedra vary between octahedra and a form some way between an octahedron and a trigonal prism. The vast majority of compositions give incommensurately modulated structures with enormous unit cells. As in the case of the other modulated phases, and the many more not mentioned, composition variation is accommodated without recourse to defects.  

ReOj cubic a = 0.37518nm, Z= 1 space group, Pm3m (No. 221)  

The oxyfluorides NbO F (a = 0.38854nm), TaO F (a=0.3896nm) and TiOF (a = 0.38102) are similar. In these cases the O and F atoms are placed at random over the oxygen sites. Both ReOj and NbOjF display octahedral tilting and a drop in symmetry when subjected to high pressure. 

Tungsten trioxide, WO3, is similar to ReOj, but the octahedra now exhibit cation displacement and octahedral tilting so that a variety of temperature-sensitive lower symmetry forms occur. Below -43°C the e-structure is monocUnic, with an a b c tilt system coupled with displacement of the W cations away from the centres of the octahedra. Between -43 and -tl7°C the 5-form has a triclinic structure with the 

The compounds known as tungsten bronzes , with a general formula A WOj, are so called because, when they were first discovered by Wohler in 1837, their metallic lustre and high electrical conductivity lead him to believe that he had made alloys of tungsten rather than new oxides. The tungsten bronzes adopt several structures, depending upon the foreign metal and the overall composition of the phase. 

---

Titanium IV) oxide, T1O2. See titanium dioxide. Dissolves in concentrated alkali hydroxides to give titanates. Mixed metal oxides, many of commercial importance, are formed by TiOj. CaTiOj is perovskite. BaTiOa, per-ovskite related structure, is piezoelectric and is used in transducers in ultrasonic apparatus and gramophone pickups and also as a polishing compound. Other mixed oxides have the il-menite structure (e.g. FeTiOj) and the spinel structure (e.g. MgjTiO ). 

There are available from experiment, for such reactions, measurements of rates and the familiar Arrhenius parameters and, much more rarely, the temperature coefficients of the latter. The theories which we use, to relate structure to the ability to take part in reactions, provide static models of reactants or transition states which quite neglect thermal energy. Enthalpies of activation at zero temperature would evidently be the quantities in terms of which to discuss these descriptions, but they are unknown and we must enquire which of the experimentally available quantities is most appropriately used for this purpose. 

THEORIES RELATING STRUCTURE AND REACTIVITY 7.2.1 The electronic theory of organic chemistry... 

A steroid very closely related structurally to cholesterol is its 7 dehydro derivative 7 Dehydrocholesterol is formed by enzymatic oxidation of cholesterol and has a conju gated diene unit m its B ring 7 Dehydrocholesterol is present m the tissues of the skin where it is transformed to vitamin D3 by a sunlight induced photochemical reaction... 

I. Lerche and J. J. O Brien, eds.. Dynamical Geology of Salt and Related Structures, Academic Press, Inc., Orlando, Pla., 1987. 

It is likely that the quinone methide and related structures formed at these temperatures account for the dark colour of phenolic compression mouldings. It is to be noted that cast phenol-formaldehyde resins, which are hardened at much... 

Since the characteristic grouping of the resins discussed in this chapter largely disappears on cross-linking it is difficult to make simple generalisations relating structure to properties. 

Harris, P.J.F. (1999) Carbon Manotubes and Related Structures Mew Materials for the Twenty-first Century (Cambridge University Press, Cambridge). 

At the present time, use of the Fischer convention is almost entirely restricted to carbohydrates, amino acids, and biologically important molecules of closed related structural types. The problem with more general use is that there are no adequate rules for deciding whether a diiral atom is like D-glyceraldehyde or L-glyceraldehyde when the structures are not closely similar to the reference molecules. This relationship is clear for carbohydrates and amino acids. 

Many completely conjugated hydrocarbons can be built up from the annulenes and related structural fragments. Scheme 9.2 gives the structures, names, and stabilization energies of a variety of such hydrocarbons. Derivatives of these hydrocarbons having heteroatoms in place of one or more carbon atoms constitute another important class of organic compounds. 

Forsberg, C. W., D. L. Moses, E. B. Lewis, R. Gibson, R. Pearson, W. J. Reich, G. A. Murphy, R. H. Staunton, and W. E. Kohn (1989). Proposed and Existing Passive and Inherent Safety-Related Structures, Systems, and Components (Building Blocks) for Advanced Light Water Reactors. Oak Ridge, TN Oak Ridge National Laboratory. 

High-resolution transmission electron microscopy (HREM) is the technique best suited for the structural characterization of nanometer-sized graphitic particles. In-situ processing of fullerene-related structures may be performed, and it has been shown that carbonaceous materials transform themselves into quasi-spherical onion-like graphitic particles under the effect of intense electron irradiation[l 1],... 

The proliferation of acidity functions is a consequence of the activity coefficient cancellation assumption. According to Eq. (8-89), a plot of log(cB/cBH+) against Hq should be linear with unit slope. Such plots are usually linear (for bases of closely related structure), but the slopes often differ from unity. - This behavior is an indication that the cancellation assumption (also called the zero-order approximation) is not valid, and several groups have devised alternatives. We will use the symbolism of Cox and Yates. ... 

The regular octahedral structure of SFg and the related structure of S2F]q (Fig. 15.20) call for little comment except to note the staggered (D4d) arrangement of the two sets of Feq in S2F10 and the unusually long S-S distance, both features presumably reflecting interatomic repulsion between the F atoms. SFg is also of... 

The isomorphous diiodides of Ce, Pr and Gd stand apart from all the other, salt-like, dihalides. These three, like LaH, are notable for their metallic lustre and very high conductivities and are best formulated as (Ln ,2I",e", the electron being in a delocalized conduction band. Besides the dihalides, other reduced species have been obtained such as LnsCln (Ln = Sm, Gd, Ho). They have fluorite-related structures (p. 118) in which the anionic sublattice is partially rearranged to accommodate additional anions. 

In order to fully appreciate the widespread application that molecular modeling can find in beginning organic chemistry, it is important to appreciate the fundamental relationship between molecular structure and chemical, physical and biological properties. So-called structure-property relationships are explored in nearly every college chemistry course, whether introductory or advanced. Students are first taught about the structures of molecules, and are then taught how to relate structure to molecular properties. 

During studies on the acid-induced formation of the alkaloid yuehchukene and related structures from pienylindoles, it was discovered that treatment of indole 173 with p-toluenesulfonic acid and silica gel leads to the formation of indolo-[3,2-b]carbazole 174 and the indole derivative 175 (Scheme 21). Similarly, the disubstituted product 176 could be isolated after TFA treatment of the substrate 177. Detailed mechanistic explanations have also been provided in this work (96T9455). 

Two other alkaloids 47 and 48 were isolated in addition to Neooxygambirtannine 44 (67T3129). Anhydroalstonatine from Alstonia venenata is a closely related structure (77IJC(B)183) (Scheme 18). 

Tetronic acids exist predominantly in the dioxo form (32) in solvents of low polarity, while the existence of the monoenol form (33) has been established in other solvents by infrared " and ultraviolet spectral comparisons- and from dipole moment data. " Haynes and Pliramer " have recently reviewed the structure of these compounds [see also reference 28(a)], and the tautomerism of vitamin A (34), which has a related structure, has also been surveyed.- Analogous compounds carrying an amino group in the 3-position are also know n. ... 

Spirapril (37) is a clinically active antihypertensive agent closely related structurally and mechanistically to enalapril. Various syntheses are reported with the synthesis of the substituted proline portion being the key to the methods. This is prepared fkim l-carbobenzyloxy-4-oxopro-line methyl ester (33) by reaction with ethanedithiol and catalytic tosic acid. The product (34) is deprotected with 20% HBr to methyl l,4-dithia-7-azospiro[4.4 nonane-8-carboxylate (35), Condensation of this with N-carbobenzyloxy-L-alanyl-N-hydroxysuccinate leads to the dipeptide ester which is deblocked to 36 by hydrolysis with NaOH and then treatment with 20% HBr. The conclusion of the synthesis of spirapril (37) follows with the standard reductive alkylation [11]. 

Guidance on specifications is divided into universal tests/criteria which are considered generally applicable to all new substances/products and specific tests/criteria which may need to be addressed on a case-by-case basis when they have an impact on the quality for batch control. Tests are expected to follow the ICH guideline on analytical validation (Section 13.5.4). Identification of the drug substance is included in the universal category, and such a test must be able discriminate between compounds of closely related structure which are likely to be present. It is acknowledged here that optically active substances may need specific identification testing or performance of a chiral assay in addition to this requirement. 

Although urushiol possesses an interesting structure for transformation into speciality polymers, no attempt has been reported. Notwithstanding its applications in a specified area, it appears that it is not properly put to use as it can be converted to polymers with better properties. The possibilities for such conversions into high-performance polymers are illustrated by cardanol, a phenolic lipid of related structure obtained from Ana-cardium occidentale. 

---