Cation geometric structures

Molecular sieves (zeolites) are artificially prepared aluminosilicates of alXali metals. The most common types for gas chromatography are molecular sieve 5A, a calcium aluminosilicate with an effective pore diameter of 0.5 nm, and molecular sieve 13X, a sodium aluminosilicate with an effective pore diameter of 1 nm. The molecular sieves have a tunnel-liXe pore structure with the pore size being dependent on the geometrical structure of the zeolite and the size of the cation. The pores are essentially microporous as the cross-sectional diameter of the channels is of similar dimensions to those of small molecules. This also contrilsutes to the enormous surface area of these materials. Two features primarily govern retention on molecular sieves. The size of the analyte idiich determines whether it can enter the porous... 

On the other hand, the crystal structures of ionic compounds with small molecular ions depend mainly on how space can be filled most efficiently by the ions, following the principle of cations around anions and anions around cations. Geometric factors such as the relative size of the ions and the shape of molecular ions are of prime importance. More details are given in Chapter 7. 

The unique properties of the proton have been attributed by some authors to the fact that it has no electronic or geometric structure. The absence of any electron shell implies that it will have a radius that is about 105 times smaller than any other cation and that there will be no repulsive interactions between electron clouds as a proton approaches another reactant species. The lack of any geometric or electronic structure also implies that there will not be any steric limitations with regard to orientation of the proton. However, it still must attack the other reactant molecule at the appropriate site. 

Calculations of electronic and geometrical structures of 1,3,2-dithiazolyl cations and corresponding radicals performed in the decade 1980-90 were reviewed in CHEC-II(1996) <1996CHEC-II(4)433> and showed that radicals are more stable than cations. Special attention was given to the association of radicals to dimers. The stabilization energy of the 1,3,2-dioxazolyl cations was found to be lower than that of the radicals, and therefore cations are more stable. 

The interpretation of the spectroscopic data on Ph-W and Ph-An clusters and cluster cations has been greatly facilitated by ab initio electronic structure calculations. Most of the calculations have been concerned with the electronic and geometric structures of the electronic ground state of the neutral clusters (see ref. [41] and references cited therein)... 

Remember that no resonance form has an independent existence A compound has characteristics of all its resonance forms at the same time, but it does not resonate among them. The p orbitals of all three carbon atoms must be parallel to have simultaneous pi bonding overlap between Cl and C2 and between C2 and C3. The geometric structure of the allyl system is shown in Figure 15-10. The allyl cation, the allyl radical, and the allyl anion all have this same geometric structure, differing only in the number of pi electrons. 

Geometric structure of the allyl cation, allyl radical, and allyl anion. 

This pronounced change in the UCST indicates a strong interaction between the two polymers. The addition of the polycation PDMAPAA-Q until R = 0.17 (where R signifies the molar ratio of polyelectrolyte/polyDMAPS) decreases the UCST first from 65 to 15 °C. The mixtures with the cationic ionenes show a similar evolution of the UCST, but also exhibit a minimum at K = 0.1-0.17. Thus, when polycations are added to 23b, the UCST decreases first, then passes through a minimum and increases again. This was attributed to the different geometrical structure of polyelectrolyte complexes (Scheme 19). 

The geometric structures of cation radicals are often considered to be similar to those of the corresponding neutral molecules from which they are generated. Such... 

E. Sanchez Marcos, R.R. Pappalardo and D. Rinaldi, Effects of the solvent reaction field on the geometrical structures of hexahydrate metallic cations, J. Phys. Chem., 95 (1991) 8928-8932. 

Five enol ethers are produced, two unrearranged (41), one 1,2-phenyl-shifted (42) and two 1,2-methyl-shifted (43). Unlike the corresponding thermal reaction (see Scheme 31 and Table 11), the product patterns are barely affected by the geometrical structure of 40 the ratio 41/42 -h 43 = 2.0 for -40 and 1.8 for Z-40. This indicates formation of a common intermediate, the primary vinyl cation 49, from both stereoisomers (Scheme 55). This cation gives 41 and rearranges to the more stable vinylic cations 46-48, which in turn give 42 and 43. 

Figure 18. Oxygen packing model of unit cell of faujasite-type framework of zeolites X ana Y, showing packing of T.O. and D-6 ting groups (left) and solid geometric representation of structure (right). Dark balls represent sodium cations in structure.

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