Dimer cations

The mechanism of ion polymerization in formaldehyde crystals proposed by Basilevskii et al. [1982] rests on Semenov s [1960] assumption that solid-phase chain reactions are possible when the arrangement of the reactants in the crystal prepares the configuration of the future chain. The monomer crystals capable of low-temperature polymerization fulfill this condition. In the initial equilibrium state the monomer molecules are located in the lattice sites and the creation of a chemical bond requires surmounting a high barrier. However, upon creation of the primary dimer cation, the active center shifts to the intersite, and the barrier for the addition of the next link... 

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... 

The first-formed cation (42) can add to a second molecule of 2-methylpropene (41) to yield the new (dimeric) cation (43) this in turn can lose a proton to yield the C8 alkene (44) or, alternatively, add to a third molecule of alkene to yield the (trimeric) cation (45), and so on. 

The "dimer-cations" Me3Si SiMe3+, R3P-PR3+, etc., which can be classified as o, and o radicals respectively, are of particular importance and hence are discussed separately. 

Finally, radical cations can be generated in solution by different types of pulse radiolysis225. Like PET, this is inherently a method for transient spectroscopic observations, but it has proved to be invaluable in investigations of dimer cations, e.g of polyenes, which form spontaneously upon diffusion of radical cations in the presence of an excess of the neutral parent compound, but a discussion of the electronic structure of such species is beyond the scope of this review. Pulse radiolysis is of interest in the present context because it allows the observation of large carotenoid radical cations which are difficult to create in solid-state or gas-phase experiments... 

The values for the ionization and protonation constants (0.21 and 0.24 respectively) indicate that [Mo205(OH2)6]2+ is the major dimeric species in 1.0 M acid with about equal concentrations of the other two dimers. The dimeric cationic species show a characteristic absorption band in the UV at —245 nm. 

Fig. 39). The ESR data for dimethylselenide dimer cation radical 107c are consistent with two-center three-electron bonding, in which two electrons are in a cr bond between the two chalcogens, while the third electron is in a o orbital. 

Alike metallocomplex anion-radicals, cation-radicals of odd-electron structure exhibit enforced reactivity. Thus, the 17-electron cyclopentadienyl dicarbonyl cobalt cation-radical [CoCp(CO)2] undergoes an unusual organometallic chemical reaction with the neutral parent complex. The reaction leads to [Co2Cp2(CO)4]. This dimeric cation-radical contains a metal-metal bond unsupported by bridging ligands. The Co—Co bond happens to be robust and persists in all further transformations of the binuclear cation-radical (Nafady et al. 2006). 

The cyclodimerization depicted in Scheme 7.19 is one of the many examples concerning cation-radicals in the synthesis and reactions of cyclobutanes. An authoritative review by Bauld (2005) considers the problem in detail. Dimerization is attained through the addition of an olefin cation-radical to an olefin in its neutral form one chain ends by a one-electron reduction of the cyclic dimer cation-radical. Unreacted phenylvinyl ether acts as a one-electron donor and the transformation continues. Up to 500 units fall per one cation-radical. The reaction has an order of 0.5 and 1.5 with respect to the initiator and monomer, respectively (Bauld et al. 1987). Such orders are usual for branched-chain reactions. In this case, cyclodimerization involves the following steps ... 

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...

Bauerle P, Segelbacher U, Maier A, Mehring M (1993) Electronic-structure of monomeric and dimeric cation radicals in end-capped oligothiophenes. J Am Chem Soc 115 10217... 

Under suitable conditions, further oxidation of the coupled products produces isolable dimer cation radicals. Thus treatment of 9-alkylcarbazoles with lead tetracetate in acetic acid-perchloric acid, or with 2,3-dichloro-5,6-dicyano-p-benzoquinone in acetic acid-perchloric acid, or with tris-(p-bromophenyl)ammoniumyl perchlorate in methylene chloride, or with nitrosonium borofluoride in acetonitrile all gave isolable cation radical perchlorates such as 17. These were reducible with aqueous sodium dithi-onite to the corresponding bicarbazoles the dimer cation radicals could be produced again by reoxidation of the dimer using 2,3-dichloro-5,6-dicyano-p-benzoquinone in acid solution. ... 

As seen from above, the mode of electron trapping in sc CO2 cannot be deduced from the results obtained in the gas phase or matrix isolation studies. It is not obvious whether the solvent radical anion should be similar to multimer cluster anions found in the gas phase, dimer cation(s) in solid matrices, or monomer CO2 anions in inert liquids. Such a situation is typical for other molecular liquids. 

The reader may notice that only saturated hydrocarbons (with a possible exception of CCI4) have been observed to yield rapidly migrating solvent holes. As mentioned above, part of this bias is explained by the fact that the holes are usually short-lived, so their dynamic properties are difficult to study. However, in many liquids (such as aromatic hydrocarbons and sc CO2), the solvent holes are relatively stable, yet no rapid hole hopping is observed. In such liquids, the solvent hole has a well-defined dimer cation core with strong binding between the two halves (in the first place, it is this dimerization that... 

Recently, by using improved nanosecond pulse radiolysis with the monitoring wavelength region from 300 to 1600 nm [44], absorption spectra due to main reactive intermediates such as the intramolecular dimer cation radical in the near-IR wavelength region were clearly observed in the pulse radiolysis of polystyrene in various solutions [47]. For example. Fig. 1 shows the absorption spectra observed in the pulse radiolysis of polystyrene solutions in CH2CI2. 

A pulse radiolysis of Ag solution was studied, and the behavior of formed silver atom (Ag ) and dimer cation (AgJ) was measured [80]. The absorption band for dimer shows a significant red shift with increasing temperatures, which implies to the CTTS (charge transfer to solvent) character of the band. 

Two classes of systems illustrate cases for which heterolytic bond dissociation lies lower than the homolytic products. The first involves transition metal dimer cations, M2+. Especially for metals to the right side of the periodic table, such cations can be considered to have ground-state electron configurations with o2dndn+1 character, where the d electrons are not heavily involved in the bonding and the o bond is formed primarily from the metal atom s orbitals. If the o bond is homolytically broken, one forms X + Y = M (s1dn+1)... 

Intermolecular a-dimer cations are known for thioethers but dimeric amine cation species are not well characterized in solution. However, an example was detected in a zeohte the confinement in the narrow channels and the restricted diffusion favor interaction between the two entities. 

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