Cation/cationic

In edge- and face-sharing, and in mutual sharing by highly charged cations, cation-cation repulsions may operate to displace the cations, if small, from the centers of their coordinated polyhedra. 

While the network equations give predictions of the distances between bonded atoms, they say nothing about the distances between non-bonded atoms. These clearly become an important factor in modelling when two anions or two cations are brought too close together. Anion-anion repulsions are important when the two anions are strongly bonded to the same cation cation-cation repulsions need to be considered when two cation coordination spheres share edges or faces. 

In 1973 Kenneth Sanborn Pitzer (1914-1997) imdertook an attempt to take into accoimt these interactions in the solution s composition. He included binary interaction cation-anion, anion-anion, cation-cation, cation-neutral component, anion-neutral component, neutral component-neutral component and triple interaction cation-cation-anion, anion-anion-cation, etc., for which he expanded first member of equation (1.78) into a series of addends with virial coefficients (Pitzer, 1973). Each of these addends characterizes one type of interaction. His model of more detailed accounting of the interaction between components of water solution is sometimes called the Pitzer model. According to it, equation (1.78) acquired the format of a virial equation of the state of solution, or Pitzer equation with virial coefficients ... 

Fig. 8 The temperature dependence of confined ion potential energy, which consists of cation-cation, cation-anion, and anion-anion intermolecular interactions, cation-wall and anion- wall interactions. Insets show radial snapshots of encapsulated ionic liquids in nanopores before melting and after melting is complete. Red and blue balls represent center-of-mass locations of bmim andPFe , respectively.

The values of rate constants for oxidation of rosaniline hydrochloride in mixed micellar solutions (CTABr + Tween-20, CTABr + Tween-80) are less than those in pure CTABr solution but are higher than those in pure nonionic surfactant solutions, whereas the values of rate constants in mixed micellar solutions (SDS + Triton X-100, SDS + Triton X-102) are less than those in pure anionic as well as in nonionic surfactants. The effects of mixed micelles of cationic-cationic, cationic-nonionic, and anionic-nonionic surfactants on the rate of alkaline hydrolysis of A-phenylbenzohydroxamic acid have been studied at 55°C where the addition of cationic surfactant to nonionic surfactant accelerates the rate of hydrolysis, and the kinetic data have been analyzed by the Monger s enzyme-kinetic-type model. ... 

Cation Cation diameter, E Crown ether Cavity diameter, E... 

Crystal stmcture analyses of cyanine and related dyes are reviewed in Ref. 32. Most typical sensitizers are nearly planar, with angles of less than 15° between planes defined by heterocycHc rings. Distinct solvent of crystallization is present in most of the cationic dyes. X-ray crystal analyses also provide intermolecular data. Because of photographic use of cyanine and carbocyanine dyes, the cation-cation arrangements of most interest have been those for l,l -dieth5l-2,2 -quinocyanine chloride [2402-42-8] 5,5, 6,6 -tetrachloro-l,l, 3,3 -tetraethylbenzimidazolocarbocyanineiodide [3520-43-2] and 5,5 -dichloro-3,3, 9-triethylthiacarbocyanine bromide [18426-56-7] (32) (see Fig. 8). 

Ionic liquids with wealdy coordinating, inert anions (such as [(CF3S02)2N] , [BFJ , or [PFg] under anhydrous conditions) and inert cations (cations that do not coordinate to the catalyst themselves, nor form species that coordinate to the catalyst under the reaction conditions used) can be looked on as innocent solvents in transition metal catalysis. In these cases, the role of the ionic liquid is solely to provide a more or less polar, more or less weakly coordinating medium for the transition metal catalyst, but which additionally offers special solubility for feedstock and products. 

It is furthermore to be anticipated that the cation-cation repulsion will operate in some cases to displace the cations from the centers of their coordinated polyhedra. This action will be large only in case the radius ratio approaches the lower limit for stability, so that the size of the polyhedron is partially determined by the characteristic anion-anion repulsive 21 Linus Pauling, Z. Krist., 67, 377 (1928). 

Simple cations are unknown within Group 16 (besides Po), but several highly colored polyatomic cations (cationic clusters), like S " ", Sg, Se, SCg, Te, and Teg" ", have been isolated in non-aqueous media [15]. Some mixed chalcogen cationic clusters have also been reported. These are all unstable in water. 

Polyanionic compounds. Frequently, the M atoms lose all their valence electrons to the X atoms, i.e. no cation-cation bonds occur and no nonbonding electrons remain at the cations, fe(MM) = 0 and E = 0. Equation (13.5) then becomes ... 

Although two closed-shell metal cations with the same charge would normally be expected to repel each other, evidence has been obtained for an entire family of cation-cation interactions in dlh or s2 systems.570 This attraction is now shown to originate from dispersion (van der Waals)... 

The overwhelming number of examples dealing with domino processes are those where the different steps are from the same category, such as cationic/ cationic or transition metal/transition metal-catalyzed domino processes, which we term homo domino processes . An example of the former reaction is the synthesis of progesterone (see Scheme 0.3), and for the latter the synthesis of vitamin E (Scheme 0.7). 

Scheme 1.1. General scheme of a cationic-cationic domino process.

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