Benzene cations

Geometries, hyperfme structure, and relative stabilities of the different positional isomers of monodeuterated benzene cations have been studied theoretically by density functional theory, using the B3-LYP functional, and experimentally by ESR and ENDOR spectroscopy. A comparison between theoretical and experimental results at 30 K gives acceptable agreement, but further experiments on multiply deuterated species should improve the analysis by making the effects of deuteration larger. 

There are some interesting aspects of the results which deserve additional study. A close scrutiny of Table 3 reveals that the difference in ZPVE between the two isomers of the mono-deuterated benzene cation does only to a very minor extent (0.5 cm ) stem from the C—H (C—D) stretching vibrations, as one would... 

Additional experimental studies on multiply deuterated benzene cations would give more information, by enlarging the effects on the ZPVE and also by introducing new structural features in the experimental spectra which can facilitate their interpretation. This would enable a more detailed and more accurate analysis, both theoretically and experimentally. Such experiments will hopefully be carried out in the near future. 

Allyl (27, 60, 119-125) and benzyl (26, 27, 60, 121, 125-133) radicals have been studied intensively. Other theoretical studies have concerned pentadienyl (60,124), triphenylmethyl-type radicals (27), odd polyenes and odd a,w-diphenylpolyenes (60), radicals of the benzyl and phenalenyl types (60), cyclohexadienyl and a-hydronaphthyl (134), radical ions of nonalternant hydrocarbons (11, 135), radical anions derived from nitroso- and nitrobenzene, benzonitrile, and four polycyanobenzenes (10), anilino and phenoxyl radicals (130), tetramethyl-p-phenylenediamine radical cation (56), tetracyanoquinodi-methane radical anion (62), perfluoro-2,l,3-benzoselenadiazole radical anion (136), 0-protonated neutral aromatic ketyl radicals (137), benzene cation (138), benzene anion (139-141), paracyclophane radical anion (141), sulfur-containing conjugated radicals (142), nitrogen-containing violenes (143), and p-semi-quinones (17, 144, 145). Some representative results are presented in Figure 12. 

New synthetic transformations are highly dependent on the dynamics of the contact ion pair, as well as reactivity of the individual radical ions. For example, the electron-transfer paradigm is most efficient with those organic donors yielding highly unstable cation radicals that undergo rapid unimolecular reactions. Thus, the hexamethyl(Dewar)benzene cation radical that is generated either via CT activation of the [D, A] complex with tropylium cation,74... 

The benzene molecule is a system of rich n electrons. Removal of n electron requires low energy and benzene cation is formed easily. Its positive charge is delocalized by the n molecular orbitals. Due to this resonance stabilization, further fragmentation of benzene cation requires considerable energy and therefore occurs with low probability. 

Model II C B X Internal-Conversion Process in the Benzene Cation... 

Parameters of Model II, Which Represents a Three-State Eive-Mode Model of the Ultrafast C — B — X Internal-Conversion Process in the Benzene Cation [179, 180] ... 

Figure 2. Diabatic (left) and adiabatic (right) population probabiUties of the C (fuU line), B (dotted line), and X (dashed line) electronic states as obtained for Model II, which represents a three-state five-mode model of the benzene cation. Shown are (A) exact quantum calculations of Ref. 180, as well as mean-field-trajectory results [(B), (E)] and surface-hopping results [(C),(D),(F),(G)]. The latter are obtained either directly from the electronic coefficients [(C),(F)] or from binned coefficients [(D),(G)].

Let us next turn to Model II, representing the C —> B —> X internal-conversion process in the benzene cation. Figure 2 demonstrates that this (compared to the electronic two-state model, Model I) more complicated process is difficult to describe with a MFT ansatz. Although the method is seen to catch the initial fast C —> B decay quite accurately and can also qualitatively reproduce the oscillations of the diabatic populations of the C- and B-state, it essentially fails to reproduce the subsequent internal conversion to the electronic X-state. Jn particular, the MFT method predicts a too-slow population transfer from the C- and B-state to the electronic ground state. 

Let us turn to Model 11 describing the C —> B —> X internal-conversion of the benzene cation. Figure 2 shows the diabatic population probabilities pertaining... 

The excellent performance of the mapping formulation for this model encouraged us to consider an extended model of the benzene cation, for which no quantum reference calculations are available [227]. The model comprises 16 vibrational DoF and five coupled potential-energy surfaces, thus accounting for... 

Figure 25. Diabatic and adiabatic population probabilities of the C (fuU line), B (dotted hne), and X (dashed line) electronic states as obtained for a five-state 16-mode model of the benzene cation.

If the snlfate anion-radical is bonnd to the snrface of a catalyst (sulfated zirconia), it is capable of generating the cation-radicals of benzene and tolnene (Timoshok et al. 1996). Conversion of benzene on snlfated zirconia was narrowly stndied in a batch reactor under mild conditions (100°C, 30 min contact) (Farcasiu et al. 1996, Ghencin and Farcasin 1996a, 1996b). The proven mechanism consists of a one-electron transfer from benzene to the catalyst, with the formation of the benzene cation-radical and the sulfate radical on the catalytic snrface. This ion-radical pair combines to give a snrface combination of sulfite phenyl ester with rednced snlfated zirconia. The ester eventually gives rise to phenol (Scheme 1.45). Coking is not essential for the reaction shown in Scheme 1.45. Oxidation completely resumes the activity of the worked-out catalyst. 

Synthetic Suitability of (Dialkylamino)benzene Cation-Radicals... 

Gridelet, E. Eorquet, A. J. Eocht, R. Eorquet, J. C. Eeyh, B. Hydrogen Atom Eoss from the Benzene Cation. Why Is the Kinetic Energy Release So Large J. Phys. Chem. A 2006, no, 8519-8527. 

H. J. Neusser For a selected intermediate J K> state we observe a couple of Rydberg series for example, for J K, = li we can identify two series under minimum residual field conditions. When we apply a stationary electric field of 300 mV/cm, additional series appear that are coupled by the electric field. All series have different limits representing different rotational states of the benzene cation. At present we cannot say whether the coupling observed under minimum residual field conditions is induced by the small stray field or by field-free intramolecular coupling. 

When the gas stream was changed from humidified air to dry air, the amount of carbon deposits, very probably attributable to the polymeric products, increased. In this stage, as the surface hydroxyl groups were consumed, the probability for the direct reaction of hole with benzene (formation of benzene cation radical) may be increased. The benzene cation radical formed on the solid surface may react with benzene, which is one of the main steps in the polymerization.83... 

As for the anode process at comparable conditions, the yield of 2,5-dimethoxy nitro benzene depends distinctly on the electrical nature of the micelle. Namely, the yields are equal to 30% for the positively charged micelle, 40% for the negatively charged micelle, and 70% for the neutral charged micelle. The observed micellar effect corroborates the mechanism, including the dimethoxy benzene cation radical and the nitrogen dioxide radical as reacting species. 

Although attempts to prepare the benzene cation and its simple alkyl derivatives have so far been unsuccessful (Bolton and Carrington, 1961b Hulme, unpublished results) it is perhaps legitimate to consider the phenoxy radical, recently detected during the oxidation of phenol by Stone and Waters (1962), as a derivative of the benzene cation, by the same token that the anion of nitrobenzene is treated as a derivative of the benzene anion. Thus the phenoxy radical may be depicted in its zwitterionic form... 

Since alkylation greatly reduces the ionization potential of benzene derivatives it was hoped that alkylated benzene cations could be prepared by oxidation in sulphuric acid (Bolton and Carrington, 1961b Hulme, unpublished results). However, although oxidation of p-xylene... 

The lowest state of prismane (2B,) cation lies 16 kcal mol-1 above the 2B2 state of the Dewar benzene cation (at the MP2/6-31 G level). This is considerably less than the corresponding energy difference of the neutral systems (37 kcal mol-1). The ground electronic states of prismane and Dewar benzene ions do not correlate their interconversion is forbidden from both state-symmetry and orbital-symmetry considerations. The CIDNP experiments indicate, however, that the actual barrier is quite small. 

The reaction begins with the one-electron reduction of benzene to form a benzene cation radical 471 (Fig. 71) [307]. Other benzene molecules have been proposed to associate in a coordinative manner with this cation radical [307,311], As each additional benzene molecule associates the cation radical becomes further delocalized. Eventually the chain of associated benzene molecules becomes so long that the terminal benzenes have too little cation-radical character to sustain further propagation. At this point the upper limit of chain length for the original cation radical is reached. When viewed from the side, the chain of coordinatively associated benzene molecules has an appearance similar to stairs. The formation... 

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