Silicon tetrahedron

Figure C2.12.1. Origin of ion exchange capacity in zeolites. Since every oxygen atom contributes one negative charge to the tetrahedron incoriDorated in the framework, the silicon tetrahedron carries no net charge while the aluminium tetrahedron carries a net charge of-1 which is compensated by cations M.

The only solution of these equations involving integers is Mj = 4, ai = 2, m2 = 2, ai = 4. Thus about each aluminum ion there will be four oxygen ions common to two aluminum octahedra and one silicon tetrahedron, and two oxygen ions common to four octahedra.23 For both kinds of oxy-... 

In spodumene, IyiAlSi206 and petalite, LiAlSUOio, it is possible that oxygen ions (with 2s,- = 2) are common to a silicon tetrahedron, an aluminum tetrahedron and a lithium tetrahedron (the radius ratio for lithium ion is 0.33). 

The electrostatic valence rule1) is approximately satisfied, deviations of not being allowed. Sharing of polyhedra must consequently be such as to have each OH or F common to two octahedra or attached to one silicon or aluminum tetrahedron only, and each 0 common to two silicon tetrahedra, four aluminum octahedra, two octahedra and one silicon tetrahedron, two or three octahedra and one aluminum tetrahedron, etc. structures with 0= common to three silicon tetrahedra or to one silicon tetrahedron and three octahedra are not satisfactory. 

Figure 3.2. A silicon tetrahedron (left), an aluminum octahedron (middle) as a central layer in a 2 1 clay, and an aluminum octahedron (right) as a surface layer in a 1 1 clay (right). The oxygen atoms are bonded to other silicon and aluminum atoms in the clay (bonds are not intended to be shown at the correct angles). Below is a water molecule showing partially positive hydrogen atoms and partially negative oxygen atoms. Also shown are the two lone pairs of electrons on all the oxygen atoms.

When the counterion is varied from lithium to sodium to potassium, the proportion of inversion increases. The relatively covalent lithium-oxygen bond favours a retentive mechanism. However, as the metal-oxygen bond becomes more ionic, the components may function more independently, allowing attack of RO on the back face of the silicon tetrahedron while electrophilic assistance by M+ of the leaving group aids inversion of configuration. 

Figure 3.3. Schematic of a silicon tetrahedron made up of four oxygen atoms surrounding one much smaller silicon atom. In a clay crystal, each oxygen atom may be part of a two-silicon tetrahedral, of a one-silicon tetrahedron and a two-aluminum octahedral, or of a one-silicon tetrahedron and a hydrogen atom (from Taylor and Ashcroft, 1972, with permission).

In all silicate minerals formed under crustal conditions silicon is coordinated to four oxygen atoms. In high-pressure transformations, silicon commonly increases its coordination number. The longer- the Si—O distances in tetrahedral silicates the higher the pressure transformations to phases with octahedral silicon. The average Si—O bond distance for the pressure transformation is 159 pm. This distance is achieved at room temperature at pressures in all measured silicates and may be a minimum for tetrahedryl Si—O bonds 300 kbar is an upper pressure limit for the silicon tetrahedron and SOkbar is a lower pressure limit for octahedral silicon. Temperature has little effect on Si—O bond distances in either tetrahedra or octahedra... 

Scientists in this field recognize that silicon polymers that have been developed commercially constitute merely the tip of an iceberg or, possibly, a silicon tetrahedron. The bulk of the silicon industry today is, in fact, based on one polymer system, poly(dimethylsiloxane) and a large family of materials obtained by minor modifications of this polymer. Imagine the prospects for other polymer systems based on silicon ... 

J. Rebek, R. McCready, New epoxidation reagents derived from alumina and silicon. Tetrahedron Lett. 20 (1979) 4337. 

Staicture of silica glass (a) the silicon tetrahedron and (b) non-crystalline network of silicon tetrahedra. 

Under basic conditions (pH > 2.5) it is likely that water dissociates to produce nucleophilic hydroxyl anions in a rapid first step. The hydroxyl anion then attacks the silicon atom. Her [124] and Keefer [125] propose a mechanism in which hydroxyl anion displaces OR— with inversion of the silicon tetrahedron (Figure 24.14), while Pohl and Osterholz [126] favor a mechanism involving a stable five-coordinated intermediate which decays through second transition state in which any of the surrounding ligands can acquire a partial negative charge. 

Under acidic conditions, it is likely that an aUcoxide group is protonated in a rapid first step. Flectron density is withdrawn from the sihcon atom, making it more electrophilic and thus more susceptible to attack from water This results in the formation of a penta-coordinate transition state with significant Sjj2-type character. The transition state decays by displacement of an alcohol and inversion of the silicon tetrahedron (Fig. 8.7). 

For non-diffusion-limited reactions carried out in low Al/Si ratio systems, the overall rate for a proton-catalyzed reaction increases linearly with the proton concentration, as illustrated schematically in Fig. 4.4 . The rate, when normalized against the framework proton concentration, however, is a constant. When the lattice Al/Si exceeds 10%, the proton-zeolite interaction energy increases. This increase in the proton-zeolite interaction decreases the intrinsic Brpnsted acidity of the zeolite. At this concentration, the tetrahedra containing A1 start to share a silicon tetrahedron. This increases the effective negative... 

Saghatelian A, Buriak J, Lin VS-Y, Ghadiri MR (2001) Transition metal mediated surface modification of porous silicon. Tetrahedron 57 5131-5136 Sailor MJ (2012) Porous silicon in practice. Preparation, characterization and applications. Wiley, Weinheim... 

Several investigators have proposed hydrolysis mechanisms involving flank-side attack without inversion of the silicon tetrahedron [48,76]. A possible acid-catalyzed mechanism is the following ... 

The silicon tetrahedron also forms stractures called pyroxenes (or inosilicates) in which many of the tetrahedrons bond together creating chains (Figure 22.6 t). The formula unit for these chains is the SiOs unit, and the repeating unit in the stracture is... 

LaRonde FJ, Brook MA. Stereoselective reduction of ketones by histidine-alkoxysilane complexes the role of imidazole in nucleophilic substitution at silicon. Tetrahedron Lett. 1999 40 3507-3510. 

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