Tetrahedral definition

The traditional definition of a zeolite refers to microporous, crystalline, hydrated aluminosilicates with a tliree-dimensional framework consisting of comer-linked SiO or AlO tetrahedra, although today the definition is used in a much broader sense, comprising microporous crystalline solids containing a variety of elements as tetrahedral building units. The aluminosilicate-based zeolites are represented by the empirical fonmila... 

Analogous definitions and designations apply to molecules containing a chiral centre and a prochiral tetrahedral or trigonal centre. The plane containing the chiral and prochiral centres is called a diastereo-zeroplane (Y. Izumi, 1977). 

The angle formed between successive bonds along the chain backbone—0 in Fig. 1.5a-is not free to assume all values, but is fixed at a definite angle depending on the nature of the bond. For the tetrahedral angle associated with carbon-carbon single bonds, d = 109.5°. 

Hydrogen has a very low solubility in the iron lattice, which makes direct observation of the location of the hydrogen atom in the lattice very difficult. The hydrogen definitely occupies an interstitial site in the bcc iron lattice. Two such sites are normally associated with interstitial solutes in bcc structures, the tetrahedral and the octahedral sites (see Fig. 8.39). Indirect evidence suggests that hydrogen occupies the tetrahedral site. 

Model A cannot be eliminated definitely by the photographs there are, however, some points which make this model improbable. From the curve for this model the first minimum would be expected to be at least as well pronounced as the second minimum, whereas on the photographs the first minimum is not very well defined. That the qualitative appearance of the photographs supports model C rather than model A is further shown by the fact that the photographs resemble those of methyl nitrate more closely than those of carbon tetrafluoride. Some evidence is also provided by the radial distribution curve (Fig. 1), the first peak being displaced by 0.15 A. from the position expected for it for model A. For these reasons and the additional reason that it is difficult to correlate the tetrahedral configuration with an electronic structure involving only completed octets, we consider model A not to be satisfactory.7... 

Abstract We briefly underline the relevance of TS-1 catalyst for industrial applications in mild oxidation reactions using hydrogen peroxide as oxidant and review the experimental works employed over last two decades for imderstanding the structme of the Ti centers in the bare TS-1 material. After an animated and controversial debate that has lasted in the literature until 1994, several works (reviewed here in depth) have definitively assessed that Ti atoms occupy framework positions substituting a Si atom and forming tetrahedral... 

Like many other chemical concepts the concept of strain is only semi-quantitative and lacks precise definition. Molecules are considered strained if they contain internal coordinates (interatomic distances (bond lengths, distances between non-bonded atoms), bond angles, torsion angles) which deviate from values regarded as normal and strain-free . For instance, the normal bond angle at the tetra-coordinated carbon atom is close to the tetrahedral value of 109.47°. In the course of force field calculations these normal values are defined more satisfactorily, though in a somewhat different way, as force field parameters. 

Figure 2.1 Definition of (—) and (+) bonds adjacent to tetrahedral stereoisomeric center.

As an example, Figure 2.5 shows both a simple (plane) and a more advanced (tetrahedral) picture of the electronic structure around the oxygen atom. In addition to the existing six electrons in the outer orbits, oxygen requires another two electrons to be stable, i.e., according to the definition of OX and the text OX0 = -2. 

The first definitive case of nickel(II) in a tetrahedral environment was in the spinel NiCtaCh (215) the octahedral sites in the oxide lattice are preferentially occupied by chromium(III) ions, leaving only tetrahedral sites for the nickel(II) ions. 

Since the Es value is determined by the relative activation free energy from the unsaturated initial state to the saturated tetrahedral intermediate state of the ester hydrolysis, Hancock and his coworkers considered that a hyperconjugation effect of a-hydrogen may contribute to the estimate of Es values 19). To separate the hyperconjugation effect from the true steric effect , they defined the parameter E° (corrected steric) as Eq. 20, assuming that the hyperconjugation effect is proportional to the number of a-hydrogen atoms, nH. By definition, E (Me) = 0. 

Natural mica compositions of the dioctahedral, potassic types have been discussed by Velde (1965b) in connection with the solid solution between muscovite and the different celadonite mica molecules. It was shown that both natural and synthetic micas formed two distinct compositional and genetic groups. The important point made in the discussion is the definition of the term mica. The most restrictive criteria as was mentioned before is the necessity for the mineral in question to have a net charge (the sum of octahedral and tetrahedral electrostatic... 

The tetrahedral framework crystals have interesting properties. It is sometimes possible for the alkali and alkaline-earth ions to be interchanged with others in solution this property permits the zeolites to be used for softening water. The water molecules also can be removed and replaced by other molecules without the destruction of the crystal. The presence of definite available positions for occupancy by large cations or water molecules is clearly indicated by the formulas of such iso-morphous pairs as natrolite and scolecite, differing in the replacement of 2Na+ + 2H,0 by Ca++ + 3H20. 

The structures of some square planar and tetrahedral complexes (314) have been definitely ascertained by means of X-ray analysis.2259-2266 Ni—N and Ni—O bond distances in these four-coordinate complexes are in the range 190-200 and 184-190 pm respectively, and do not differ substantially on going from one complex to another. The structures of two dinuclear complexes were reported to contain both bridging and terminal salicylaldimines (315).2269,2270... 

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