Evidence for Cations

Reactions such as S l reactions in water are believed to involve cationic intermediates, but do not give sufficient concentrations of the cations for 

Larger chemical shifts for 4-f or 3C atoms of a particular type correspond to greater deshielding, caused by more positive charge at these positions 

Carbocations show characteristic H and, 3C NMR spectra with chemical shifts for the charged centres downfield of all neutral centres, due to the deshielding effect of the positive charge. 

Q Draw resonance structures for 2 and 3 to show the positions at which positive charge build-up is expected. 

Cations react rapidly and indiscriminately with nucleophiles. Thus if a reaction which is suspected to go via a cationic intermediate is carried out in the presence of an added nucleophile, and an adduct containing the new nucleophile is obtained, this provides evidence for a cationic intermediate which is trapped by the added nucleophile. For example, the addition of bromine to alkenes is thought to go via a cationic intermediate 4 (reaction 5.11). If chloride or nitrite ions are added to the reaction mixture, the chloride or nitrite adducts 5 and 6 are obtained, even though the chloride and nitrite ions do not react with ethene (or 1,2-dibromoethane) directly at a rate that would account for the amount of these products in the reaction mixture. This provides strong evidence for the intermediate bromoethyl cation 4, which will be trapped by the added chloride or nitrite ions (reactions 5.12 and 5.13). The structure of bromoalkyl cations is discussed further in the section on electrophilic addition. 

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The term "cutionic water" may be used to describe the situation in which water appears in coordination compounds apparently joined to cations by covalent bonds. However, the fact that a number of such compounds exhibit "hydrate isomerism" is evidence for cationic bonding, as well as it is lor the existence of other harms of these compounds in which Ihe presence of Water is due to electrostatic attractions or ctystal stability requirements. 

The formation of an ideal solution between two or more components requires that the configurational entropy be the maximum value, eq. (7.4). This implies that ions must be randomly distributed over coordination sites in the crystal structure. Whenever cation ordering occurs in a structure, the configurational entropy is not the ideal maximum value. The evidence for cation ordering summarized in 6.7 indicates that few silicate minerals are ideal solutions. 

Bums, R. G. (1970) Crystal field spectra and evidence for cation ordering in olivine minerals. Amer. Mineral., 55, 1608-32. 

However, all these studies were carried out under objectionable experimental conditions (absence of high vacuum) and cannot be accepted as evidence for cationic polymerization in the absence of cocatalyst. Longworth et al. (57) presented direct evidence which showed that the Russian workers were in error. 

The successful generation, by precipitation out of liquid anhydrous HF, of fluorides thermodynamically unstable with respect to loss of elemental F2, gave a forceful reminder of the remarkable stability of that solvent towards oxidation. Soon after the preparation of AgFa, in attempts to find evidence for cationic derivatives (e.g. [AgF2] ), it was discovered (see Ref. 98) that even divalent silver, as a cation, Ag + (solvated by HF), had the capability to oxidize Xe. This made chemical sense, since a cation, having an electron deficit, should have higher electronegativity than a related oxidation-state in a neutral or anionic species. Indeed, this had three important consequences. 

Newcomb, M., R. Shen, S.-Y. Choi, RT. Toy, PF. Hollenberg, A.D.N. Vaz et al. (2000). Cytochrome P450-catalyzed hydroxylation of mechanistic probes that distinguish between radicals and cations. Evidence for cationic but not for radical intermediates. J. Am. Chem. Soc. 122, 2677-2686. 

Hanzlik, R.P. and R.H. Tullman (1982). Suicidal inactivation of cytochrome P-450 by cyclopropyl-amines. Evidence for cation-radical intermediates. J. Am. Chem. Soc. 104, 2048-2050. 

Hoberg, A.-M., Haddleton, D. M., Derrick, P. J., and Scrivens, J. H., Evidence for Cationization of Polymers in the Gas Phase During Matrix-assisted Laser/ Desorption Ionization, Eur. Mass. Spectrom., 3, 471,1997. 

Hanton, S.D., Owens, K.G., Chavez-Eng, C., Hoberg, A.-M., and Derrick, P.J. (2005) Updating evidence for cationization of polymers in the gas phase during matrix-assisted laser desorption/ionization. Eur. J. Mass Spectrom., 11, 23-29. 

Baumann (154) collected data from other sources for comparison with the data of Alexander plotted in Figure 1.6 these data seem to confirm that the solubility reaches a minimum at pH 7-8, but the reason for the slightly higher solubility at lower pH is not known. However, Cherkinskii and Knyaz kova (160) verified the data (see Figure 1.6) and proposed that silica is amphoteric and is cationic below pH 7. They give equations assuming that all the soluble silica is actually polymeric and cationic at low pH. There is no experimental basis for such a tljeory all of the soluble silica has been shown to be monomeric, and there is no evidence for cationic silica above the isoelectric point of pH 2. 

Evidence for cation formation was partly derived by the observation that for t-butyl substituted carbinols, rearrangement occurs to cleanly provide the tetralin or indane. For example, either 29 or 31 reacted with 90% H2SO4 to provide the tetralin. Compound 30 resulted from either Me or H 1,2-shift."... 

Further pioneering efforts at CLIO gave spectra for metal ion complexes of small peptides [100], namely the sodium ion complexes of di- and tri-glycines and -alanines. About the same time, the group at FELIX in collaboration with our group reported results for sodium and potassium complexes of PheAla and AlaPhe, giving clear evidence for cation-n coordination of the phenyl ring [12]. 

A.2 Supporting Evidence for Cationic Active Site for Ethylene Polymerization... 

For the transition metal ions, Cr +, Fe " ", Fe +, Ni +, and Cu " have all been studied by NDIS methods, with the results attesting to the richness and uniqueness of structures these ions possess in aqueous solution. Of particular interest for these cations are results for Cu + which are strongly dependent on counterion and concentration. The exact details of the Cu + coordination remains controversial with claims for five-fold overall coordination in copper perchlorate solutions. For solutions of copper nitrate and copper perchlorate, there is no evidence for cation-anion contacts, however, in aqueous solutions of copper chloride at concentrations above three molal, there is strong evidence for copper-chloride compexation. 

Taken together, the clinical evidence for cationic lipidipDNA-mediated gene transfer and the finding of pDNA-mediated toxicities in these same vectors implies that the euiient therapeutic index of these vectors will require signifieant improvement to be usefiil. 

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