Silicon atomic size

The oxidation state -1-4 is predominantly covalent and the stability of compounds with this oxidation state generally decreases with increasing atomic size (Figure 8.1). It is the most stable oxidation state for silicon, germanium and tin, but for lead the oxidation state +4 is found to be less stable than oxidation state +2 and hence lead(IV) compounds have oxidising properties (for example, see p. 194). 

Aluminum distribution in zeolites is also important to the catalytic activity. An inbalance in charge between the silicon atoms in the zeolite framework creates active sites, which determine the predominant reactivity and selectivity of FCC catalyst. Selectivity and octane performance are correlated with unit cell size, which in turn can be correlated with the number of aluminum atoms in the zeolite framework. ... 

Unit Cell Size (UCS). The UCS is a measure of aluminum sites or the total potential acidity per unit cell. The negatively-charged aluminum atoms are sources of active sites in the zeolite. Silicon atoms do not... 

Explain why the size of the silicon atom does not permit a silicon analog of the graphite structure. 

Bearing in mind that the relative sizes of the ions permit isomorphous replacement of OH by F but not by Cl, we write with considerable confidence the formula (Si, Al, Fe, P)18O20(OH, F)laCl, which agrees well with analyses 1, 2, and 3. Inasmuch as aluminium (as well as phosphorus) may replace silicon with coordination number 4, it is evident that there are at least five silicon atoms in the unit, corresponding to the chemical formula... 

Stability towards heating, irradiation and exposure to air was also reported for [n]pericyclinosilanes 70-72 [20, 21], and even the highly strained 74 [24]. But, in contrast to the hydrocarbons, the pericyclinosilanes did not react with bromine [21 ]. On the other hand, the presence of the silicon atoms brings about a specific reactivity of these macrocycles making them labile and capable of changes in ring size. Thus, when a mixture of compounds 70-72 a (n = 6) was... 

Why does siiicon not form itt bonds as readiiy as carbon, its Group 14 neighbor The answer can be found by examining the sizes of the vaience orbitais of siiicon and carbon. Figure 10-23 compares side-by-side p-orbital overiap for carbon atoms and siiicon atoms. Notice that for the larger silicon atom, the 3 orbitals are too far apart for strong side-by-side overiap. The resuit is a very weak ttbond. Consequently, silicon makes more effective use of its vaience orbitais to form four a bonds, which can be described using s p hybrid orbitals. 

When the steric size of the substituents increases, no adducts with bulky (Et3N) or less basic (EtzO) donors are formed with (Me3C)2Si= NSi(CMe3)3.14 The structure of the benzophenone adduct is quite remarkable.16 Instead of the expected cycloaddition product, the carbonyl oxygen atom is coordinated to the undersaturated silicon atom. 

No effect of this type is manifested for the addition of alkyl radicals to the same alkenes. Evidently, the steric effect involved in the addition of trialkylsilyl radicals to 1,2-disubstituted ethylene derivatives is due to the repulsion between the carbon and silicon atom, caused by the large size of the silicon atom in the reaction center of the transition state. 

It has already been mentioned that the formation of ultrastable Y zeolites has been related to the expulsion of A1 from the framework into the zeolite cages in the presence of steam (8,9), and the filling of framework vacancies by silicon atoms (11,12). This results in a smaller unit cell size and lower ion- exchange capacity (6). It also results in a shift of X-ray diffraction peaks to higher 20 values. Ultrastable Y zeolites prepared with two calcination steps (USY-B) have a more silicious framework than those prepared with a single calcination step (USY-A). Furthermore, since fewer aluminum atoms are left in the USY-B framework, its unit cell size and ion-exchange capacity are also lower and most of the nonframework aluminum is in neutral form (18). 

The difference in the properties of the silicon atom from that of the carbon is because of its comparatively bigger size, less electronegative nature and the presence of the available vacant d orbitals. 

Some vibrational modes of zeolites are sensitive to the amount of aluminum in the framework [93]. The substitution of aluminum for silicon atoms in the zeolite framework changes the T-O-T bond angles (where T is a tetrahedral atom that can be either Si or Al). This is primarily due to the smaller size and different charge density of the aluminum atoms compared to silicon. This results in a shift in frequency for vibrational modes in the zeolite due to external linkages. The T-O-T asymmetric (1100-980 cm ) and symmetric (800-600 cm ) stretching modes as well as structural unit vibrations Mke double four- and double six-rings exhibit a shift to lower frequencies as the aluminum content of the framework is increased. Figure 4.19 shows this relationship for a faujasite-type framework. 

Boron trifluoride etherate was used in conjunction with the reducing agent borane to rearrange aromatic O-triisopropylsilyl ketoximes to cyclic and acyclic aniline derivatives. The steric hindrance of the substituents on the silicon atom, the size of the aliphatic ring and the presence of alkoxy substituents on the aryl group played important roles in the aniline formation. 

A further enhanced degree of reduction leads to chains of group 14 elements with Ne values between 6 and 6.6 e/atom. These values cover the range between twofold bound silicon atoms in one-dimensionally extended zigzag chains and 2 dumbbells. Isolated chains reside in between these border structures and therefore compete with them [14]. This leads to the expectation of an extreme cation size dependence of the anion formation. Table 4 contains the reported isolated homoa-tomic chains in solid state compounds. 

Very recently, a mixture of silicon and tin reduced by barium metal yielded the compound BagSisSn, which contains three-atom rings of silicon atoms Sig and monoatomic Sn" [85]. This shows that the combination of differently sized anionic atoms gives the opportunity to expand into hitherto unknown (Vg r(M)) regions that contain new poly anions. 

The distribution of cations in a hydrated zeolite is mainly controlled by their sizes and can be described by a statistical model. In the dehydrated state, most of the cations are located on the intraframework sites their occupancies are governed by mutual repulsions and cation—framework interactions [1]. By which, the environments of the framework silicon atoms and their corresponding ssi NMR spectra are affected [2,3]. The chemical shift and lineshape of Si NMR have been found to depend on the nature and the distribution of cations in the small sodalite and double hexagonal prism (D6R) cavities of the dehydrated Y zeolites [3] The irreversible migration of La3 ions from the supercages to the small sodalite and/or D6R cavities by... 

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