Anion cation association

Generally, MIRC reactions lead to racemic mixtures of cyclopropanes. The stereoselectivity of the reaction is determined by the cyclization step on the in situ formed intermediate Michael adduct. The cisjtrans ratio of the substituents on this newly formed bond depends on the solvent polarity, the degree of anion-cation association and steric interactions. When the reaction takes place under phase-transfer conditions the cisltrans ratio also depends on whether or not a catalyst is used. ... 

What is the stmcture of neat ionic liquids. Even though the stmcture of liquids, such as water, has been studied for many years, the study of room-temperature ionic liquids is still in its infancy [58]. Purified [BMIM][PFj], probably the most studied IL, has been shown to be purely monophasic, with no aggregates, but to have a local stmcture [47]. Imidazolium groups are positioned in pairs with a plane-to-plane separation of 4.5 A. The stmcture of ILs is also characterized by a degree of anion-cation association. Thus, in [EMIM][NTf2] and [BP][NTf2], the self diffusion coefficients of the anions and the cations measured by pulsed-gradient spin-echo N M R are superior to the diffusion coefficients deduced from the molar conductivity, which demonstrates the existence of ion pairs which cannot contribute to the ionic conduction [53]. 

Thus, in contrast to an ionization process from a neutral substrate, which initially generates an intimate ion pair, deamination reactions generate a cation which does not have an anion closely associated with it. Furthermore, the leaving group, molecular nitrogen, is very stable so that little, if any, nutleophilic participation is needed for bond cleavage. The... 

A catalyst is defined as a substance that influences the rate or the direction of a chemical reaction without being consumed. Homogeneous catalytic processes are where the catalyst is dissolved in a liquid reaction medium. The varieties of chemical species that may act as homogeneous catalysts include anions, cations, neutral species, enzymes, and association complexes. In acid-base catalysis, one step in the reaction mechanism consists of a proton transfer between the catalyst and the substrate. The protonated reactant species or intermediate further reacts with either another species in the solution or by a decomposition process. Table 1-1 shows typical reactions of an acid-base catalysis. An example of an acid-base catalysis in solution is hydrolysis of esters by acids. 

MC has a much stronger influence on ion-pair dissociation than PC. The EO units on MC coordinate cations which have been dissociated by the carbonate group, and prevent cation association with the anion. It is thought that, whereas conventional plasticizers like PC create fast ion-... 

Preparative chromatographic resolution procedures have overall freed chemists from the constraint of dependency on crystallization. They are most often performed with covalent diastereomer mixtures but ionic salts can also be separated. Recently, it was found that the lipophilicity of TRISPHAT anion 8 profoundly modifies the chromatographic properties of the cations associated with it and the resulting ion pairs are usually poorly retained on polar chromatographic phases (Si02, AI2O3) [131]. Using enantiopure TRISPHAT anion. 

Homogeneous catalytic processes are those in which the catalyst is dissolved in a liquid reaction medium. There are a variety of chemical species that may act as homogeneous catalysts (e.g., anions, cations, neutral species, association complexes, and enzymes). All such reactions appear to involve a chemical interaction between the catalyst and the substrate (the substance undergoing reaction). The bulk of the material in this section will focus on acid-base and enzyme catalysis. Students interested in learning more about these subjects and other aspects of homogeneous catalysis should consult appropriate texts (11-12, 16-29) or the original literature. 

In a more detailed study, it was shown that MW effects are strongly dependent on the temperature and the nature of the cation associated with hydroxide anion [66] (for example Eq. (46) and Tab. 5.20). 

This association has its counterpart that was also variously described as an encounter complex, a nonbonded electron donor-acceptor (EDA) complex, a precursor complex, and a contact charge-transfer complex.10 For electrically charged species such as anion/cation pairs (which are relevant to ion-pair annihilation), the pre-equilibrium association results in contact ion pairs (CIP)7 (equation 3)... 

Likewise, cationic acceptors afford mixed (positively) charged complexes with electron-rich donors,11 i.e., [D, A+] and anionic donors associate with electron-poor acceptors to form mixed (negatively) charged complexes,12 i.e., [D-, A]. In each case, the intermolecular (ionic) complexation or association represents the highly oriented organization of the donor/acceptor pair (independently of whether they bear positive, negative or no charge) that is often sufficient to afford crystalline complexes amenable to direct X-ray structure elucidation.13... 

The consequence of these partial charges is that one surface of kaolinite is compatible with and attractive to the other surface. This results in increased stability of kaolinite and the formation of relatively stable structures. Some kaolinite particles can be larger than the 0.002 mm upper limit for clay Both surfaces also attract and hold water through these partial charges. The absorptive activity of kaolinite is associated with its surface electrons and partially positive hydrogens, and thus the two faces of kaolinite can attract anions, cations, water, and electrophilic and nucleophilic organic compounds. 

Because of their importance as basic primary centers, we will now discuss the optical bands associated with the F centers in alkali halide crystals. The simplest approximation is to consider the F center - that is, an electron trapped in a vacancy (see Figure 6.12) - as an electron confined inside a rigid cubic box of dimension 2a, where a is the anion-cation distance (the Cr -Na+ distance in NaCl). Solving for the energy levels of such an electron is a common problem in quantum mechanics. The energy levels are given by... 

In the case of a few relevant anions we found a good linear correlation between the hydration enthalpies in the gas phase and the hydration numbers when these anions are associated with quaternary cations under PTC conditions (19). The hydration numbers measured for OH (11.0+1) and F (8.5+1) are in good agreement with those previously extrapolated, i.e. OH (10.0) and F (9.4) (20,21). 

Aqueous cation C and anion A) associated into ion pair C Aj... 

In the case of alkali metals, ion pairing can be visualized as HFCs from paramagnetic nuclei of the metal cations associated with organic anion-radicals in ethereal solvents. In this respect, an alkali metal cation associated with the anion-radical of o-dimesitoylbenzene in DME or THF serves as a paradigm (Herold et al. 1965). 

The structure of ulexite [26] is based on the isolated pentaborate trianion shown in Fig. 21. This polyborate anion is associated with one crystallograph-ically unique 6-coordinate Na" cation and one unique 8-coordinate Ca " cation. The cations form chains by sharing coordinated oxygen, and these chains link the polyanions together into parallel sheets. These sheets are associated by coordination with interstitial Ca " cations and hydrogen-bonding. 

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