Cation comparison

Similarly to the 2-norbornyl cation, comparison of calculated (IGLO/DZ//MP2/ 6-31G(d)) and experimental 13C NMR chemical shifts allowed to differentiate between the hypercoordinated 70 and the trivalent form 71 of the bicyclo[2.1.1.]hexyl cation.85 The experimental (157.8 ppm) and calculated (158.5 ppm) values for Cl and C2 (averaged signal) are reported to be nearly identical for the symmetrically bridged... 

The problem arises, which equilibrium constant offers the most effective measure of carbocation stability A good discussion of this question has been provided by Mayr and Ofial,29 who point out that a rigorous comparison of stabilities is possible only for isomeric cations. Comparisons between nonisomeric cations depend on the equilibrium chosen for the measurements. They argue that the appropriate choice depends on the context and imply that it is not possible to identify a best measure of carbocation stability. While this is certainly true it is worthwhile pursuing further the likelihood that some equilibria provide better measures of stability than others, and to assess their effectiveness and limitations. 

In contrast to the protonation of the disubstituted alkenes mentioned above, high ktranJkcis ratios of 9 x 108 and 3 x 103, respectively have been observed in the acid-catalyzed addition of methanol to the trans- and cis-isomers of cycloheptene (39a and 38a) and cyclooctene (37a and 36a) (114a). The rate constants reflect partially the release of strain in the transformation of the cyclic olefins to the appropriate cycloalkyl cations. Comparison of the relative activation energies for these addition reactions with the difference of strain release leads to the estimate that the response to strain effects is about 60%. In a more recent study of the acid-catalyzed hydration of cis- (36b) and trans- 1-methylcyclooctene (37c), it was concluded that two conformationally different 1-methyl-carbocationic intermediates are... 

Nelson s detailed investigation of one-electron oxidation of bicyclic amines and hydrazines continues CV studies show that /V -diacyl-hydrazines (19)30a and azabicyclooctanes (20a)30b gjVe long-lived radical cations comparison has been made between ease of electron loss and N inversion barriers in (20b) and related N-chloroamines 30c studies of the nitroxides (21) and related compounds show that ff-coupied transmission of positive charge from N to the CO in the radical cation is unimportant. 30d... 

For an earlier study, see Apeloig, Y., Schleyer, P. v. R., Pople, J. A. (1977). Molecular orbital theory of the electronic structure of molecules. 35. p -Substituent effects on the stabilities of ethyl and vinyl cations. Comparison with isoelectronic methyl boranes. The relative importance of hyperconjugative and inductive effects. Journal of the American Chemical Society, 99,5901-5909. 

D. 0. Leitch, C., and Bechgaard, K. (1974). Mixed-Valence Diferrocenylacetylene Cation. J. Am. Chem. Soc., 96,6788-6789. (g) McManis, G. E. Gochev, A. Nielson, R. M., and Weaver, M. J. (1989). Solvent Effects on Intervalence Electron-Transfer Energies for Biferrocene Cations Comparisons with Molecular Models of Solvent Reorganization. J. Phys. Chem., 93, 7733-7739. (h) Delgado-Pena, F. Talham, D. R., and Cowan, D. 0. (1983). Near-IR Spectroscopic Studies of Mixed-Valence Di-, Tri-, and Tetraferrocene Derivatives. J. Organomet. Chem., 253, C43-C46. (i) Powers, M. J. Meyer, T. J. (1978). Intervalence Transfer in Mixed-Valence Biferrocene Ions. J Am. Chem. Soc., 100, 4393-4398. 

Jones et al. [144,214] used direct dynamics with semiempirical electronic wave functions to study electron transfer in cyclic polyene radical cations. Semiempirical methods have the advantage that they are cheap, and so a number of trajectories can be run for up to 50 atoms. Accuracy is of course sacrificed in comparison to CASSCF techniques, but for many organic molecules semiempirical methods are known to perform adequately. 

In this section the influence of micelles of cetyltrimethylammonium bromide (CTAB), sodium dodecylsulfate (SDS) and dodecyl heptaoxyethylene ether (C12E7) on the Diels-Alder reaction of 5.1a-g with 5.2 in the absence of Lewis-add catalysts is described (see Scheme 5.1). Note that the dienophiles can be divided into nonionic (5.1a-e), anionic (5.If) and cationic (5.1g) species. A comparison of the effect of nonionic (C12E7), anionic (SDS) and cationic (CTAB) micelles on the rates of their reaction with 5.2 will assess of the importance of electrostatic interactions in micellar catalysis or inhibition. 

As mentioned, we also carried out IR studies (a fast vibrational spectroscopy) early in our work on carbocations. In our studies of the norbornyl cation we obtained Raman spectra as well, although at the time it was not possible to theoretically calculate the spectra. Comparison with model compounds (the 2-norbornyl system and nortri-cyclane, respectively) indicated the symmetrical, bridged nature of the ion. In recent years, Sunko and Schleyer were able, using the since-developed Fourier transform-infrared (FT-IR) method, to obtain the spectrum of the norbornyl cation and to compare it with the theoretically calculated one. Again, it was rewarding that their data were in excellent accord with our earlier work. 

Comparison of the rate of nitration of the base with that of a necessarily cationic derivative. 

We shall now consider the implications of these newer results for the nitration of these cations, taking first the comparison of the anilinium ion with its increasingly methylated homologues, then the various cations containing the trimethylammonio group, and finally cations containing elements other than nitrogen. 

Kinetic data are available for the nitration of a series of p-alkylphenyl trimethylammonium ions over a range of acidities in sulphuric acid. - The following table shows how p-methyl and p-tert-h xty augment the reactivity of the position ortho to them. Comparison with table 9.1 shows how very much more powerfully both the methyl and the tert-butyl group assist substitution into these strongly deactivated cations than they do at the o-positions in toluene and ferf-butylbenzene. Analysis of these results, and comparison with those for chlorination and bromination, shows that even in these highly deactivated cations, as in the nitration of alkylbenzenes ( 9.1.1), the alkyl groups still release electrons in the inductive order. In view of the comparisons just... 

For this series of compounds qualitative information is quite extensive. Application of the criteria discussed in 8.2, in particular comparison with the corresponding methyl quaternary salt, establishment of the rate profile for nitration in sulphuric acid, and consideration of the encounter rate and activation parameters, shows that 2,4,6-collidine is nitrated as its cation. The same is true for the 3-nitration of 2,4- ... 

The interest attaching to the nitration of pyridine i-oxide and its derivatives has already been mentioned ( 8.2.5). Some data for these reactions are given in tables 8.1, 8.2 and 8.4. The 4-nitration of pyridine I-oxide is shown to occur through the free base by comparison with the case of i-methoxypyridinium cation ( 8.2.2), by the nature of the rate profile ( 8.2.1), and by consideration of the encounter rate ( 8.2.3). - Some of these criteria have been used to show that the same is true for... 

With the oxides which are nitrated as the cations the difficulties are much less serious for the use of an acidity function is not involved. Comparison of 2,6-dimethoxy- and 3,5-dimethoxy-pyridine i-oxide with wt-dimethoxybenzene (which is nitrated at the encounter rate)... 

The nitration of phenylpyridines and related compounds has attracted attention for a long time, and measurements of isomer proportions have been made for several compounds of this type. Nitration occurs in the phenyl ring. For 2-phenylpyridine and 2-phenylpyridine i-oxide measurements of the dependence of rate of nitration upon acidity in 75-81 % sulphuric acid at 25 °C show that both compounds are nitrated as their cations (table 8.1). The isomer distribution did not depend significantly upon the acidity, and by comparison with the kinetic data for quinolinium ( 10.4.2) the partial rate factors illustrated below were obtained.They should be compared with those for the nitration of 2-nitrobiphenyl ( 10.1). The protonated heterocyclic groups are much... 

Among the W-oxides of this series of compounds isoquinoline 2-oxide shows the simplest behaviour on nitration. The acidity dependence of the rate of nitration (table 8.1), and comparison with the 2-methoxyiso-quinolinium cation (v, R = Me) (table 10.3) show the oxide to be nitrated as its conjugate acid (v, i = H) in 76-83 % sulphuric acid. The... 

Calcium hydride has also been used as the base[2,3]. A comparison of the effect of metal cations indicated that yields increase in the order < Na < Li and a procedure in which -BuLi serves as the base has been developed [4]. 

FIGURE 4 14 Electro static potential maps of methyl cation and ethyl cation The region of high est positive charge is more concentrated in CH3 and more spread out in CH3CH2 (The electrostatic potentials were mapped on the same scale in order to allow direct comparison )... 

The PTCR effect is complex and not fully understood in terms of the grain boundary states and stmcture. Both the PTCR effect and room temperature resistivities are also highly dependent on dopant type and ionic radius. Figure 11 (32) illustrates this dependence where comparison of the PTCR behavior and resistivity are made for near optimum concentrations of La ", Nd ", and ions separately substituted into BaTiO. As seen, lowest dopant concentration and room temperature resistivity are obtained for the larger radius cation (La " ), but thePTCR effect was sharpest for the smallest radius cation (Y " ), reflecting dual site occupancy of the Y " ion. 

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