Chemistry counterions

FIG. 3 Condensed DNA particle assembly using template polymerization of cations, (a) Principle of DNA condensation by counterion polymerization on DNA template (b) chemistry of polycation formation (c) e microscopy of condensed DNA particles, bar =100 nm. [(c) was reprinted from Ref. 97, copyright 1998 Oxford University Press.]... 

Random coil conformations can range from the spherical contracted state to the fully extended cylindrical or rod-like form. The conformation adopted depends on the charge on the polyion and the effect of the counterions. When the charge is low the conformation is that of a contracted random coil. As the charge increases the chains extend under the influence of mutually repulsive forces to a rod-like form (Jacobsen, 1962). Thus, as a weak polyelectrolyte acid is neutralized, its conformation changes from that of a compact random coil to an extended chain. For example poly(acrylic acid), degree of polymerization 1000, adopts a spherical form with a radius of 20 nm at low pH. As neutralization proceeds the polyion first extends spherically and then becomes rod-like with a maximum extension of 250 nm (Oosawa, 1971). These pH-dependent conformational changes are important to the chemistry of polyelectrolyte cements. 

Begala, A. J. Strauss, U. P. (1972). Dilatometric studies of counterion binding by polycarboxylates. Journal of Physical Chemistry, 76, 254-60. 

Rymden, R. Stilbs, P. (1985a). Counterion self-diffusion in aqueous solutions of poly(acrylic acid) and poly(methacrylic acid). Journal of Physical Chemistry, 89, 2425-8. 

In the present case, the electron hopping chemistry in the polymeric porphyrins is an especially rich topic because we can manipulate the axial coordination of the porphyrin, to learn how electron self exchange rates respond to axial coordination, and because we can compare the self exchange rates of the different redox couples of a given metallotetraphenylporphyrin polymer. To measure these chemical effects, and avoid potentially competing kinetic phenomena associated with mobilities of the electroneutrality-required counterions in the polymers, we chose a steady state measurement technique based on the sandwich electrode microstructure (19). 

Another example of how the hard-soft interaction principle applies to precipitation can be seen in a familiar case from analytical chemistry. Because ions of similar size and magnitude of charges precipitate (interact) best, a good counterion for precipitation of Ba2+ is one that is of similar size and has a —2 charge. In accord with this, Ba2+ is normally precipitated as the sulfate because of the favorable size and charge of the anion compared to the cation. 

The application of the HSAB principle is of considerable importance in preparative coordination chemistry in that some complexes are stable only when they are precipitated using a counterion conforming to the above rule. For example, CuCls3 is not stable in aqueous solution but can be isolated as [Cr(NH3)6][CuCl5]. Attempts to isolate solid compounds containing the complex ion Ni(CN)s3 as K3[Ni(CN)5] lead to KCN and K2[Ni(CN)4]. It was found, however, that when counterions such as Cr(NH3)63+ or Cr(en)33+ were used, solids containing the Ni(CN)53 anion were obtained. 

Influence of Metal Ions on Oxygen Chemisorption and Ignition of Chars. We have carried out extensive studies of the influence of metal ions in wood on pyrolysis mechanisms (5.6) and this approach has now been extended to oxygen chemisorption of the chars. The metal ions occur in wood predominantly as the counterions of the uronic acid components of the hemicelluloses (12). We have shown that they can be almost completely removed by very mild acid treatment without any other major change in the chemistry of the wood. Table II shows that the major metal ions in cottonwood are Ca, K and Mg. The acid-washing process removed 98X of the metal ions in... 

Much confusion in the early history of aqueous transition-metal chemistry stemmed from the inability to distinguish free formula ions (serving merely as solvent-separated counterions) from those that remain in direct coordinated contact with... 

A remarkable feature of iridium enantioselective hydrogenation is the promotion of the reaction by large non-coordinating anions [73]. This has been the subject of considerable activity (anticipated in an earlier study by Osborn and coworkers) on the effects of the counterion in Rh enantioselective hydrogenation [74]. The iridium chemistry was motivated by initial synthetic limitations. With PFg as counterion to the ligated Ir cation, the reaction ceases after a limited number of turnovers because of catalyst deactivation. The mechanism of... 

Cationic zirconocenes serve as useful reagents in such diverse fields as alkene polymerization, carbohydrate chemistry, asymmetric catalysis, and so on. Reagents that were originally developed for polymerization reactions (MAO, ansa-metallocenes, non-nucleophi-lic borate counterions) have now found use in organic synthesis and are being employed for carbometalation reactions, hydrogenation, and Diels—Alder catalysis. 

Two important developments in the past couple of decades now allow us to better study coordination chemistry (of ions) in the gas phase, without the intrusion of the normally ever present solvent molecule or a counterion. 

Sulfur has four unique characteristics related to its occurrence and chemistry in soil. As sulfate, it is one of the principle counterions that keep the soil electrically neutral. Soil receives constant additions of sulfur through volcanic activity around the world and industrial pollution, usually in the form of acid rain. This means that soils usually have sufficient sulfur for plant growth. Lastly, plants can take and use sulfur dioxide from the air as a source of sulfur for growth [22,38],... 

The effect of the counterion binding reactions 23 and 24 on the surface chemistry will be treated in terms of ratios p and n, defined as ... 

Co/pH and V o/pH results are sensitive to different aspects of the surface chemistry of oxides. Surface charge data allow the determination of the parameters which describe counterion complexation. Surface potential data allow the determination of the ratio /3 —< slaDL- Given assumptions about the magnitude of the site density Ns and the Stern capacitance C t, this quantity can be combined with the pHp2C to yield values of Ka and Ka2. Surface charge/pH data contain direct information about the counterion adsorption capacitances in their slope. To find the equilibrium constants for adsorption, a plot such as those in Figures 7 and 8 can be used, provided that Ka and Kai are independently known from V o/pH curves. 

It was previously mentioned was that a large number of minor copolymers of PET have been developed over the past 50 years, with the intent of modifying textile fiber properties and processability [2, 3], Of broader interest is that some of these textile modifications, such as PET copolymers with metal salts of 5-sulfoisophthalic acid (SIPA), have their own rich chemistries when the extent of polymer modification is increased beyond textile levels. An example of such a modification is that changing the counterions associated with SIPA can significantly effect the kinetics of polyester transesterification reactions (the... 

In this section, an example will be given in which a (small) library of a new type of cationic lipids was synthesized and screened for TE (63). For synthesis, combinatorial solid phase chemistry was used. All cationic lipids of the example library are structurally based on 3-methylamino-1,2-dihydroxy-propane as the polar, cationic lipid part. As nonpolar lipid part, different hydrocarbon chains are boimd to the amino group of the scaffold and the amino group was further methylated to get constantly cationic-charged lipids. Lipids were synthesized in both configurations and as racemats, and the counterions were varied as well. Table 1 summarizes the structural features of these lipids. 

The synthesis, chemistry, and complexing behavior of phospholide (6, with 6a, 6b, and 6c resonance structures) and polyphospholide anions have been reviewed recently.The [C H Ps J series with n = 0—4 is a complete set of structures with a successive replacement of CH units by the same heteroelement, P. The counterions are alkaline metals (e.g., Ps K+, which, together with K2HP7, has been obtained from red P in refluxing DMF in a yield of... 

It is evident from the preceding discussion that some, but not all, reaction mechanisms are sensitive to the level of solvation present. What of the counterion, the. oppositely charged ion in solution often regarded merely as a spectator In ICR spectrometry, only one type of ion, positive or negative, is normally trapped in the cell at a time. The chemistry of ions independent of any counterion can thus be examined. 

An example where the presence of a counterion makes a difference between the gas phase and solution phase pathways involves the intriguing carbanion produced on deprotonation of 1,3-dithiane at C-2. In solution, this species, almost invariably produced by reaction of the dithiane with butyllithium, is widely used as an acyl anion equivalent in synthetic chemistry. Its importance for the present work is that this is a configurationally stable lithiated species in solution the carbanion stays sp -hybridized, and the lithium prefers the equatorial position, even to the extent of driving a terr-butyl group on the same acidic C-2 carbanion to the axial position in the lithiocarbon species. The carbanion is thought to be stabilized primarily by orbital overlap with the C-S antibonding orbitals, as opposed to more conventional polar and 7t-resonance stabilization. ... 

ION-MOLECULE CHEMISTRY THE ROLES OF INTRINSIC STRUCTURE, SOLVATION, AND COUNTERIONS John E. Bartmess... 

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