Benzene dimeric cation

Table 5. Binding Energies Ei, for the Methyl-Substituted Benzene Dimer Cations Based on Application of the Generic Standard Hydrocarbon Model to the Radiative Association Kinetic Data. I obs is the Efficiency per Collision of Radiative...

Both silica and alumina have served as a host for oxidation of benzene by ultraviolet irradiation, leading to the benzene dimer cation radical (benzene) (Tanei, 1968). Photoionization here is thought to be biphotonic (p. 180). On the other hand, the formation of perylene and anthracene cation radicals on silica alumina is enhanced by ultraviolet irradiation, and the process is found to be monophotonic (Takimoto and Miura, 1972). The fate of the photo-ejected electron is, of course, not known, a state of ignorance which pertains to all of the cation-radical forming reactions on catalyst surfaces. 

Another type of interaction is the association of radical ions with the parent compounds. Recently (118), a theoretical study was reported on the interaction of butadiene ions with butadiene. Assuming a sandwich structure for the complex, the potential curve based on an extended Hiickel calculation for two approaching butadienes (B + B) revealed only repulsion, as expected, while the curves for B + and B + B" interactions exhibit shallow minima (.068 and. 048 eV) at an interplanar distance of about 3.4 A. From CNDO/2 calculations, adopting the parameter set of Wiberg (161), the dimer cation radical, BJ, appears to be. 132 eV more stable than the separate B and B species, whereas the separate B and B species are favored by. 116eV over the dimer anion radical, BJ. This finding is consistent with experimental results formation of the dimer cation radical was proved in a convincing manner (162) while the attempts to detect the dimer anion radical have been unsuccessful. With other hydrocarbons, the reported formation of benzene dimer anion radical (163) represents an exceptional case, while the dimeric cation radical was observed... 

Many other ion-molecule reactions involving highly unsaturated hydrocarbon ions and neutral olefins or the equivalent strained cycloalkanes have been studied by mass spectrometry98. For example, we may mention here the addition of ionized cyclopropane and cyclobutane to benzene radical cations giving the respective n-alkylbenzene ions but also isomeric cyclodiene ions such as ionized 8,9-dihydroindane and 9,10-dihydrotetralin, respectively. Extensive studies have been performed on the dimerization product of charged and neutral styrene4. 

Simple MO considerations show that the dimer cation of an aromatic hydrocarbon M, with one less antibonding electron than the ground excimer configuration, should be stable with respect to its constituents M+ + M this is confirmed by esr129 and optical absorption8 studies of y-irradiated solutions of benzene, naphthalene, and anthracene in low temperature glasses. 

Fluorescence detected magnetic resonance effects observed during the pulse radiolysis of anthracene-d10 in the presence of tetramethylethene portray an additional facet of the cyclobutane radical cation system [345, 346]. The spectra [eight ( ) equivalent methyl groups, ad = 8.2 G approximately one half of the monomer splitting, am = 17.1 G] are compatible with a dimer cation. In analogy to the benzene dimer radical cation [347, 348] they were interpreted as evidence for a sandwich , one molecule above the other [346],... 

In cyclohexane geminate recombination occurs very efficiently and the observation of polymer ions is rather difficult [57, 58]. However, when the electron scavenger such as chloroform and carbon tetrachloride was added to the solution of polystyrene in cyclohexane, a weak, broad absorption band with a maximum at lOOOnm due to dimer cation of benzene was observed. The dimer cation radical might be produced by the hole migration, along the polymer chain, from a radical cation to a site suitable for the dimer-cation formation [59]. 

Jahn-Teller distorted state. ENDOR results of dimeric benzene cation formed after warming in CFCI3 and present in CF3CCI3 at all temperatures indicate that the dimer had a sandwich structure in both matrices. An EPR study of the benzene radical cation in argon matrix generated by fast electron irradiation at 16 K showed favorable stabilization of big state rather than b2g state, in contrast to previous results found in Freon matrices [180]. 

In the continuation of our work a study has also been made of the system benzene/silica gel. When irradiating this system at 77 °K. it was found that the silica gel could stabilize both monomeric and dimeric cation radicals of benzene (6). Furthermore, the high resolution of the electron spin resonance lines indicated a high degree of mobility for the benzene molecules in the adsorbed layer. No spectrum from trapped electrons could be observed although this could very well be hidden behind the strong cation absorption. However, ethylene, and isobutylene in the adsorbed state at low temperature gave spectra from shortlived species identified as trapped electrons (7). 

Figure 2. Radiolysis/EPR result after reaction of 1,3-cyclohexadiene on H-Beta. a) Radiolysis at 77 K gave the benzene radical cation, a = 4.5 G. b) Annealing the sample above approximately 120 K caused the transformation of benzene radical cations to benzene dimer radical cations, a = 2.2 G.

Y. A. Mantz, F. L. Gervasio, T. Laino, and M. Parrinello,/. Phys. Chem. A, 111, 105-112 (2007). Charge Localization in Stacked Radical Cation DNA Base Pairs and the Benzene Dimer Studied by Self-Interaction Corrected Density-Functional Theory. 

Fig. 2 Dissociation curve of cationic benzene dimer calculated with DFTB and DFTB-CI approaches, figure adapted from ref. 56. Experimental values belong to the green band (refs. 57-63).

Experimental results suggest that PCl is dimeric in CCl solution. The stmcture consists of two octahedra sharing edges (56). PCl is monomeric in benzene and apparendy is trigonal bipyramidal (36). SoHd PCl is ionic, consisting of [PCl cations and [PClg ] anions (36). 

In weaker acid systems, other reactions involving the triplet state supervene to the exclusion of dimerization. Photolysis of 85 in 3-3% sulfuric acid, 96-5% acetic acid, and 0-2% water gave as products tri-phenylmethane (93), 9-phenylfluorene (94), 6is-9-phenylfluorenyl peroxide (95) and benzophenone (96). When benzene was present, tetra-phenylmethane (97) was also formed in addition to the other products. When the triphenylmethyl cation is irradiated in 3-3% H2SO4, 80 1% HOAc, 16-4% toluene, and 0-2% H2O, the products observed were... 

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