Counter-Ion Effects

It is also difficult to determine exactly the relative stabilities of vinyl cations and the analogous saturated carbonium ions. The relative rates of solvolysis of vinyl substrates and their analogous saturated derivatives have been estimated to be 10 to 10 (131, 134, 140, 154) in favor of the saturated substrates. These rate differences, however, do not accurately reflect the inherent differences in stability between vinyl cations and the analogous carbonium ions, for they include effects that result from the differences in ground states between reactants, as well as possible differences between the intermediate ions resulting from differences in solvation, counter-ion effects, etc. The same difficulties apply in the attempt to estimate relative ion stabilities from relative rates of electrophilic additions to acetylenes and olefins, (218), or from relative rates of homopropargylic and homoallylic solvolysis. 

There are many papers which purport to record the effect of counter-ion on such factors as transfer constants, co-polymerisation ratios, etc. It is significant that in most of these studies relatively high initiator concentrations have been used, so that counter-ion effects are more likely but before accepting that the observed effects are indeed due to change of counter-ion (derived from different catalysts or co-catalysts) it must be ascertained that these polymerisations are in fact cationic and not pseudo-cationic - in which case the effects would stem from the different reactivities of different esters (see Section 5). 

For reactions in solvents of much lower DC, such as chloroform or benzene, the situation is of course entirely different, because then ion-pairs, and perhaps higher aggregates, will dominate the reaction pattern, and in such circumstances counter-ion effects are of course quite plausible. However, at present we have no experimental information which would be of use in a more precise analysis of such systems. 

DR. SUTIN Various values for the cobalt-nitrogen distances in the hexaammines have been reported. Part of this variation is almost certainly due to lattice (counter-ion) effects. Recent literature values for the Co-N distances in... 

Diastereoselective Rhodium(l)-Catalyzed Intramolecular [4-r2] Counter-Ion Effect... 

Variations in the absolute concentration of the carbocation solutions and temperature had minor effects on chemical shifts. The counter ion effect and the equilibrium could be minimized by going to higher superacidity systems with lower nucleophilicity counter ions. Resonances due to the PAH itself were considerably shielded. Solvation by FSO3H and the formation of ion pair-molecule clusters were suggested as possible reasons. 

Figure 7 shows a plot of x as a function of cp. A rough correlation can be seen if cp is positive then x is positive and if cp is negative then x is negative. The crystal structure of XAGWUX contains two independent molecules with opposite conformations. One of the molecules is syn-syn the other is anti-anti. This occurrence frustrates any attempts to correlate the conformation of the cation with the acidic strength of the anion or other counter ion effects. 

Investigation of environmental effects. As has been stressed in this chapter, homoaromaticity is just a matter of a few kcal mol-1 stabilization energy in most cases, and therefore environmental effects may have a large impact on structure, stability and other properties of a homoaromatic compound. Future work in theory (as well as in experiment) has to clarify how environmental effects can influence electron delocalization, through-space interactions and bonding in homoaromatic molecules. The theoretical methods are now available to calculate solvent and counter ion effects (for homoaromatic ions in solution) or to study intermolecular and crystal packing forces in the solid state. 

For ionic complexes, or compounds carrying polar groups, hydrogen bonding to polar solvent molecules may occur and influence the shielding of the metal nucleus. This has been demonstrated for [V(CO)6]. This anion is subjected to a solvent isotope effect of —0.60 ppm per in D2O, which compares with —0.01 per in the non-polar toluene- fg [33]. A counter-ion effect of the same order of magnitude has also been noted. 

Baleizao C, Pires N, Gigante B et al (2004) Friedel-Crafts reactions in ionic liquids the counter-ion effect on the dealkylation and acylation of methyl dehydroabietate. Tetrahedron Lett 45 4375-4377... 

Because ESI is less tolerant of sample conditions, sample preparation is even more critical than in MALDI. In addition, the solvent used for the ESI process is extremely important, unlike in MALDI, where the solvent evaporates away before the analysis. Nonvolatile solvents like water or a 1% concentration of sodium-dodecyl-sulfate (SDS) tend to produce poor sprays and a poor or noisy ion current. A sample in 99% water will not spray as well as a sample prepared in 50% methanol. A peptide sample prepared using acetic acid will also produce better results than one using trifluoroacetic acid (TEA) probably due to a counter ion effect. Nonvolatile salts or small molecules will eventually plug the entrance hole to the mass spectrometer and so they should be avoided. 

The donor and acceptor properties of the solvent as well as its dielectric control the degree of ion association. If this effect needs to be minimized for a cationic solute, anions such as perchlorate or tetraphenylborate should be used with solvents that possess a dielectric greater than, say, 15. For an anionic complex in a polar solvent, lithium could be used as a counter ion. Effects of ion association are more noticable for complexes that carry a high charge, for example 3+, than for ions that carry a single charge. 

Generally, both types of dimeric surfactants still show well-defined, clear CMCs, though in selected cases the CMC is less obvious to standard methods, in particular to conductometric methods [415, 424, 425]. The values of CMCs determined by different methods agree well, and the counter-ion effects are the usual ones [426,427]. But, strikingly, CMC values of both bola-surfactants and of gemini-surfactants are lowered by one order of magnitude or more in... 

Fundamental studies of gas-phase ionic processes are also of interest in other areas, including combustion, the chemistry of the ionosphere, interstellar chemistry and chemical vapour deposition. Another important aspect of gas-phase studies is that they probe the intrinsic reactivity of ionic species in the absence of counter ions and solvent. Indeed, in cases where sufficient data are available, comparisons between solution- and gas-phase studies provide insights into solvent and counter ion effects. ... 

No counter ion effect is discernible for samples with other anions. The proton-coupled NMR spectrum consists of a quartet (/= 163 Hz), as expected for CH3 —Ni. The multiplicity of the NMR spectra of methene- and ethenediazenium ions (see Table 5-1) show the expected coupling for slow exchange. The triplet of doublets in the NMR spectrum of methenediazenium ion is consistent with the assigned structure. The NMR chemical shift of this ion is comparable to that of the terminal C-atoms in allene. The chemical shift and the splitting of the 2,2,2-trifluoroethanediazonium ion are consistent with its structure the shift change relative to the methanediazonium ion is considerable but fairly well understandable. 

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