Anions, cation-anion pairs

Example of copredpitation (a) schematic of a chemically adsorbed inclusion or a physically adsorbed occlusion in a crystal lattice, where C and A represent the cation-anion pair comprising the analyte and the precipitant, and 0 is the impurity (b) schematic of an occlusion by entrapment of supernatant solution (c) surface adsorption of excess C. 

The concentration of salt in physiological systems is on the order of 150 mM, which corresponds to approximately 350 water molecules for each cation-anion pair. Eor this reason, investigations of salt effects in biological systems using detailed atomic models and molecular dynamic simulations become rapidly prohibitive, and mean-field treatments based on continuum electrostatics are advantageous. Such approximations, which were pioneered by Debye and Huckel [11], are valid at moderately low ionic concentration when core-core interactions between the mobile ions can be neglected. Briefly, the spatial density throughout the solvent is assumed to depend only on the local electrostatic poten-... 

The active site is viewed as an acid-base, cation-anion pair, hence, the basicity of the catalyst depends not only on the proton affinity of the oxide ion but also on the carbanion affinity of the cation. Thus, the acidity of the cation may determine the basicity of the catalyst. Specific interactions, i.e., effects of ion structure on the strength of the interaction, are likely to be evident when the carbanions differ radically in structure when this is likely the concept of catalyst basicity should be used with caution. 

The second series of data on protic solvent effects in bromination that are related to transition states comprises the m-values of solvent-reactivity correlations. First, it is important to underline that 7-parameters, the solvent ionizing powers, established from solvolytic displacements, work fairly well in this electrophilic addition. This is expected since bromination, like SN1 reactions, leads to a cation-anion pair by heterolytic dissociation of the bromine-olefin CTC, a process similar to the ionization of halogenated or ether derivatives (Scheme 14). 

For each cation-anion pair model parameters /i J, /i J, C x, and am. 

STEP 3. For each cation-anion pair MX, evaluate functions g(x) and g (x) for x = aMxsfT,... 

Table 8.4. hmw model parameters for cation-anion pairs... 

STEP 4. The second virial coefficients for cation-anion pairs are... 

The cation-anion pair potential for a symmetrical salt may be written as the sum of the core repulsion and the coulomb interaction. For example, if the core repulsion is an inverse power potential, then ... 

The second type of interaction is dependent on the ion sizes and for the cation-anion pairs on the perturbation parameter, g. The core repulsive potential for a cation-anion pair has the form... 

Because of coulomb repulsions, the core repulsive potential between ions of like sign may be assumed to be negligibly small in the most significant configurations (see Section II-A). Consequently, in these configurations, the interaction potential between ions of like sign is a function only of the distance between the ions and is independent of the type of ions. The cation-anion pair potentials may be written as... 

The contrast between the lack of enantioselectivity in Scheme 39 and the moderate to excellent diastereoselectivity seen with alcohol nucleophiles in Schemes 19 and 33 can be attributed to the difference in leaving groups (diphenyl phosphate vs diethyl phosphate) and to the differences in the radical cations themselves, all of which impinge on the rate of equilibration of the contact alkene radical cation/anion pair. 

A series of N-allyl sulfamates, phosphoramides, and phosphorimidates was prepared to explore the possibility of O- N rearrangements via the intermediacy of the contact alkene radical cation/anion pair, followed by 5-exo-trigonal radical cyclizations (Fig. 4) [142],... 

For a pure salt dissolved in water it is not feasible to determine the activity coefficient, V , for the cation and anion separately so that the mean activity coefficient concept has been defined for a salt s cation-anion pair as... 

Activity Coefficients. The mean ionic activity coefficients of a cation-anion pair (here shortened to "activity coefficient" for convenience) are directly measureable in pure and occasionally in mixed solutions. The mean ionic activity coefficient is... 

In this development, attention is focused exclusively on activity coefficients of cation-anion pairs, with no use being made of activity coefficients of individual ions. 

The structure of the compound consists of two residues, one cationic (PYS-PYSH)+, containing the S-S bond linking the two 2-mercapto-pyridine molecules one of which is protonated and Ij as a counter anion. In the crystal lattice there are four symmetry-independent cation-anion pairs. There are only a few crystal structures reported in the literature containing open chain stable cations of DS-SD dimers, such as the monocationic [(C HgN S-SNjC Hj) ) ) [15]. The two I-I bond distances of the Ij" in the four components of complex (26) are 2.887(4) and 2.944(3) A in component a, 2.874(4) and 2.957(3) A in b, 2.968(3) and 2.862(3) A in c and 2.855(4) and 2.927(3) A in d, respectively, indicating a slight asymmetry of I3 in this complex (covalent hnear asymmetric). 

On the basis of kinetic studies, a mechanism for the radical oxidation of thioether with 36 has been proposed and is indicated in Scheme D ". The key step involves the formation of a radical cation-anion pair within the solvent cage. The presence of the pic ligand in the coordination sphere of the metal reduces the electrophilicity of the peroxo complex, thus allowing the competitive radical process to take place. 

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