Cation enhancement

This is, however, a weak electrophile compared with species such as N02 and will normally only attack highly reactive aromatic compounds such as phenols and amines it is thus without effect on the otherwise highly reactive PhOMe. Introduction of electron-withdrawing groups into the o- or p-positions of the diazonium cation enhances its electrophilic character, however, by increasing the positive charge on the diazo group ... 

Fig-1- The silicon (3-effect stabilisation of a (3-cation enhances the nudeophilicity of allyl-... 

In many reactions, transfer of the anion across the interface and subsequent diffusion into the bulk of the organic phase will not be the rate-determining step when lipophilic catalysts are used, but the effect of less lipophilic catalysts may be influenced more by the anion and the mechanism of the transfer process. Thus, for example, the reactive anion is frequently produced in base-initiated reactions by proton extraction from the substrate at the two-phase interface and diffusion of the ion-pair contributes to the overall kinetics of the reaction. Additionally, the reactivity of the anion depends on its degree of hydration and on its association with the quaternary ammonium cation. In most situations, the activity of the transferred anion is enhanced, compared with its reactivity in aqueous media, as its degree of hydration is reduced, whereas a relatively weak electrostatic interaction between the two ions resulting from the bulkiness of the cation enhances the reactivity of the anion by making it more available for reaction and will be a major factor in the ratedetermining step. 

Also called vapour-phase interferences or cation enhancement. In the air-acetylene flame, the intensity of rubidium absorption can be doubled by the addition of potassium. This is caused by ionization suppression (see Section 2.2.3), but if uncorrected will lead to substantial positive errors when the samples contain easily ionized elements and the standards do not. An example is when river water containing varying levels of sodium is to be analysed for a lithium tracer, and the standards, containing pure lithium chloride solutions, do not contain any ionization suppressor. 

Engebretson, R.R. and von Wandruszka, R. (1998) Kinetic aspects of cation-enhanced aggregation in aqueous humic acids. Environ. Sci. Technol., 32, 488-493. 

Patti G, Woo H, Yanes O, Shriver L, Thomas D, Uritboonthai W, Apon J, Steenwyk R, Manchester M, Siuzdak G (2010) Detection of carbohydrates and steroids by cation-enhanced nanostructure-initiator mass spectrometry (NIMS) for biofluid analysis and tissue imaging. Anal Chem 82 121-128. doi 10.1021/ac9014353... 

Most of these metal-oxygen intermediates are expected to be reactive toward organic substrates and electrode surfaces. Hence, the presence of metal cations enhances the electron stoichiometry for the reduction of 02, but frequently passivates the electrode surface. Thus, the formation of (H20)4Znn(02) on the surface of a platinum electrode probably initiates a metathesis reaction ... 

ATaCh Li, Na, K Excess alkali cation enhances catalytic activity. Co-catalysts not found to be essential although NiO was also used in addition in some studies. 442-446... 

Ba (Dove and Nix, 1997). Rate enhancements in single salt solutions can be predicted by equations that follow the form of Langmuir-type isotherms, whereas in mixed salt solutions, behavior follows a competitive cation-surface interaction model (Dove, 1999). Dove and co-workers argue that the alkali and alkaline earth cations enhance dissolution by modifying characteristics of the solvent at the mineral-solution interface. 

Nucleophilicity of the guanidine must be. carefully controlled to avoid arylation of the catalyst itself. This could be easily accomplished through a proper choice of the substituents. Note also that delocalization of charge over the three nitrogens in the assumed intermediate guanidinium cation enhances the nucleophilicity of its counter anion, e.g. the p.nitro phenoxide anion. 

The media represented in Table 10 contain, as a rule, a protic component but in small amounts. However, the content of proton donors can be increased appreciably hydrogenation of benzene can be effected in mixtures of water and aptotic solvents (such as diglyme, ethylenediamine, sulfolan. Table 11). Furthermore, benzene can be reduced not only in a mixture of aptotic and protic solvents but often in protic media as well. Mono- and dibasic alcohols and also water (Table 11) proved to be suitable for the purpose. Although the presence of tetrabutylammonium cations enhances to some extent the dissolution of benzene in an aqueous phase. 

Adsorption of organic solutes, control, 260 Adsorption of surfactants cation enhancements, 261 conceptual models, 261 Adsorption option in SOLMINEQ.88, description, 124—125 Adsorptive additivity, description, 272 Al, thermodynamic properties, 415 Aluminate ion... 

The presence of isolated metal cations in the zeolite channels also plays an important role in the oxidation of coke formed during the alkylation of toluene with ethylene. The facility of coke burning was characterized by the initial temperature at which the coke started to be removed as CO and CO and the temperature for the CO and CO2 concentration maxima. It appears that Mn and especially Fe cations enhance significantly coke burning. The initial temperature for coke oxidation and the concentration maxima for CO and CO evolution were 470, 590 and 780 K, resp., for FeH-ZSM-5 and 520, 765 and 830 K, resp., for pure H-ZSM-5. On the other hand, A1 cations apparently slightly retard coke oxidation (CO is evolved at higher temperatures) likely owing to steric hindrances. 

Potassium analysis is usually carried out by flame spectrometry. Atomic emission spectrometry (AES) is slightly more sensitive, though atomic absorption spectrometry (AAS) is somewhat more immune to interference. Interferences occur in the presence of high concentrations of sodium and due to the formation of refractory potassium phosphates in the flame. A solution containing 0.4 mmol cesium chloride and 0.15 mmolL lanthanum nitrate dissolved in 0.1 M HCl will reduce both cation enhancement and anionic suppression (Wieland 1992, Birch and Padgham 1993). 

CoOx may affect the adsorption of CO or O2 on R. Since at low temperatures the reaction rate on R is determined by the slow adsorption of oxygen due to CO inhibition, it is most likely that CoO serves as 0-supplier for die reaction. No influence of cobalt oxide on the CO adsorption on R was detected by IR measurements. If we assume that R-Co alloy formation does not play an important role in the CO/O2 reaction over Pt/CoOx/SiQj, several models may account for flie observed effects. According to our first model, cobalt cations enhance the adsorption of O2 on R by an increased electron back-donation into the anti-bonding orbitds of O2, which facilitates O2 dissociation. The increased back donation may be induced by the electrical field of the cobalt cations. The second model is shown schematically in figure 4. CO is adsorbed on R. O2 dissociates on CoO and the dissociation may be assisted by the presence of O-vacancies present on cobalt oxide. COa on R will react with Oa on cobalt. This reaction will then take place at the interface between R and CoOx. It is also possible that Oa migrates fi"om tiie CoO to R, in which case the reaction proceeds on the R surface (third model). The authors are in favour of the last two models since R itself is already able to dissociate O2 around 100 K if fi ee R sites are available (no CO inhibition) [33]. 

The larger radius of anions compared to cations enhances their polarizability (a v ... 

Coletta TF, Bruell CF, Ryan DK, Inyang HU. (1997). Cation-enhance removal of lead from kaolinite by electrokinetics. Environmental Engineering 123(12) 1227-1233. 

Acidic proteins elude with F and phosphate salt solutions (0.05 to 0.15 M), but not with Cr or divalent cations. This is because F and phosphate edge out the carboxyl groups of the protein from the mineral s calcium. Divalent cations enhance the binding of acidic protein to... 

Several concepts (categories of action) that could result in cation enhanced fluorescence can be envisioned. Five examples are summarized here (a) the ion could cause subtle change(s) in energy levels or electron densities that lead to enhanced fluorescence, (b) the cation of interest could displace a quencher complexed by the crown, (c) the complexation of a cation could interrupt a quenching mechanism operable in the free crown, (d) complexation could adjust the conformation so that a new fluorescent excited state might form, (e) a crown ether used in an extraction method could promote the solubility of a fluorescent ion in a phase that is monitored for fluorescence. The literature of crown ethers contains examples employing each of these concepts (i-2i) with the possible exception of case (b). Seve concq)ts we have attempted to employ follow. 

Bis(crown ether) compound 4 was synthesized in the hope that both crown ether rings would sandwich one metal ion between them, and that this process would bring the two naphthalene chromophores into close proximity so that an excited dimer (excimer) would form and a characteristic fluorescence band would appear. Early examples of metal ion fostered excimer formation were reported by Bouas-Laurent and co-workers(T) and Tundo and Fendler(S). Compound 4 has not shown excimer fluorescence, probably because the decrease in energy is small when two naphthalene chromophores form an excimer and because the 2,3-disubstitution on the naphthalene rings may present steric barriers to excimer formation. However, 4 does exhibit cation enhanced fluorescence as an extraction agent(2i) in dichloromethane(22). 

Figure 6 depicts the process of cation enhancement of fluorescence. Bis(crown ether) 4 at 2 x 10 molar concentration in the dichloromethane phase experiences intramolecular quenching. Using model compound 3 we determined that 75 percent of the fluorescence is quenched, presumably by intramolecular EDA quenching. When the metal ion in the water phase can be extracted by bis(crown ether) 4 into the dichloromethane phase, the complexation decreases the intramolecular quenching dramatically. 

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