Localized cations

Delocalized cations, represented by two or more resonance contributors, are usually more stable than localized cations. However, the fact that several resonance contributors can be drawn for a molecule does not guarantee that the molecule will actually be resonance stabilized (see also Chapter 12, Problem 10). 

Scheme 4.1 includes examples of oxymercuration reactions. Entries 1 and 2 illustrate the Markovnikov orientation under typical reaction conditions. The high exo selectivity in Entry 3 is consistent with steric approach control on a weakly bridged (or open) mercurinium ion. There is no rearrangement, indicating that the intermediate is a localized cation. 

The presence of local cation ordering in Mg2Ga and MgsGa - CO3 LDHs noted in Sect. 3.3.1 has been confirmed by means of both EXAFS and by calculation of the electron radial distribution function from the Fourier transform of the diffracted X-ray intensity. In each case the gallium was found to have six magnesium ions and no galhum ions as next-nearest neighbors [39]. 

In the case of LDH hosts, since only the average structure is known, it is important to consider the local structure by means of X-ray absorption spectroscopy extended X-ray absorption fine structure (EXAFS). According to an ideal model based on edge-sharing octahedra [64], it is possible to define the local environment around each type of cation in relation to the layer-charge density. Figure 11 shows the local cation environment for LDH with... 

The nature of size variations in zeolites during C02 adsorption is, except for NaA, similar to the size variation of the same zeolites during water adsorption. Smaller changes in the sizes (approximately by half) on adsorption of C02 are probably caused by the lower adsorption energy of C02 molecules, which is approximately 30% less than that of water. The absence of an initial expansion section for zeolite NaA is attributed to the fact that carbon dioxide molecules adsorbed equally well on cations and on the void walls, and the lower energy of interaction with the cation cannot appreciably redistribute the electron density in the voids by water molecules in interaction with non-localized cations. 

Amides are protonated on oxygen because a resonance-stabilized cation is produced. Protonation on nitrogen gives a localized cation. 

Aryl cations are highly reactive intermediates with two possible electronic configurations, the singlet and triplet states. The former is a localized cation with a vacant a orbital at the dicoordinated carbon atom, whereas the latter has a diradical character with single occupancy of the g orbital and the charge being delocalized in the... 

Accordingly, whilst five-membered heterocycles exclusively give arylation in a-position toward aryl cations, the Imposition is only arylated in 2,5-dimethyl derivatives. Such selectivity for attacking in the a-position is greater than in other electrophilic substitutions, which indicates that triplet phenyl cations are not localized cations and therefore are more selective electrophiles [53],... 

The NMR spectrum of this allylic cation reveals a plane of symmetry, which confirms that the positive charge is spread over two carbons. The large shift of 224 p.p.m. for these carbons indicates very strong deshielding (that is, lack of electrons) but is nowhere near as large as a localized cation. The middle carbon s shift of 142 p.p.m. is almost typical of a normal double bond indicating that it is neither significantly more nor less electron-rich than normal. 

In pentacoordinated systems three center-two electron bonds help stabilize the system relative to a localized cation. This becomes more... 

Solid-state NMR also allows for the study of sample preparation effects on the local cation environment. Na-SPS with 1.7% styrene sulfonate groups cast from dimethyl formamide or tetrahydrofuran (THF)/ water mixtures shows more aggregated ions, i.e., a more intense peak at 20 ppm, than ionomers cast from less... 

This differs from the singlet counterpart for the demotion of one of the tc electrons to the ct space (tc ct configuration). Thus, two (orthogonal) singly occupied orbitals give to this intermediate the character of a diradicalic rather than of a localized cation. In other terms singlet and triplet phenyl cations resemble the respective spin states of carbenes. As a result, in this multiplicity the triplet phenyl cation reacts efficiently with tc nucleophiles. 

Note that as we change crystal structures in the other alkaline earth pyrophosphates, changes in lattice symmetry, and cation coordination occur, even though the local cation symmetry remains the same, i.e.- Ci =... 

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