Dimerization radical cations

The ion-molecule reaction between thiirane and its radical cation to form a thiirane sulfide radical cation and ethylene has been studied by Qin, Meng and WiUiams [134]. ESR studies using a low-temperature sohd-state Freon radiolysis technique provided compeUing evidence that the hemibonded dimer radical cation of thiirane is an intermediate in this so-called sulfur-transfer reaction see Scheme 2. 

The lattice-stabilization effects allow the isolation of [MF6]- salts (M=As, Sb) of [S3N2]2+ in the solid state from the cycloaddition of [SN]+ and [S2N]+ cations in S02.66 The S-S and S-N bond distances in the planar, monomeric dication are shorter than those in the dimeric radical cation dimer, as anticipated for the removal of an electron from a n orbital. 

The absence of dimer radical cation formation by diphenyl selenide under the pulse radiolysis conditions is in contrast to bimolecular reactions believed to occur under electrochemical conditions/ In these experiments, a rotating disk electrode was used in combination with commutative voltammetry under anhydrous conditions. The results led to the conclusion that reversible one-electron oxidation is followed by disproportionation, then reaction of the resulting dication with diphenyl selenide or an external nucleophile, with the likely intermediacy of the dimer dication (Fig. 33). As expected, the dihydroxy selenane is formed when water is present. Based on the kinetics of the electrochemical reaction, the authors believe the diselenide dication, not the radical cation, to be the intermediate that reacts with the nucleophile. 

At higher concentrations or under annealing conditions, radical cations 106b-108b form dimer radical cations 107c, and 108c, respectively... 

Fig. 39 Radical cations and dimer radical cations from dimethyl ehalcogenides.

Intramolecular dimer radical cation and dimer dication formation... 

Just as intermolecular interactions result in dimer radical cation formation from a radical cation and a molecule of starting material, the presence of two chalcogens atoms within a molecule in proximity to one another can result in intramolecular dimer radical cation formation. 

Ditellurole (122) differs from the compounds described in the previous two sections due to its small ring size, combined with the presence of two sp -hybridized carbons (Fig. 45). As a result, formation of the intramolecular dimer radical cation... 

Oxidation of arylolefins, enolethers, or dienes yields intermolecular homocoupling products in moderate to good yield (see Sect. 13.2.1.4) however, no pronounced diastereoselectivity was observed. This is also due to the fact that the coupling sites do not tolerate substituents that would make up a prostereogenic center. Furthermore, the fairly stable cations of the dimerized radical cation solvolyze stereounselectively. The same holds for the intermolecular coupling of aromatic compounds, in... 

In the photochemical one-electron oxidation of aromatic sulfides, dimer radical cations were formed in rapid equilibrium with monomeric radical cation (59). The complex formation of a- and tt-types has been shown to be sensitive to the steric and electronic influence of substituent. For the case of jo-(methylthio)anisole the formation of TT-type dimer was shown to be reduced due to steric hindrance of two methyl groups. No formation of dimer radical cation was observed for jo-(methoxy)thioanisole and diphenyl disulfide where the corresponding monomer radical cations are stabilized by the delocalization of positive charge on the sulfur atom. Density-functional calculations supported the experimental results. The intramolecular formation of similar radical... 

Hydroxyl radicals were generated radiolytically in NaO-saturated aqueous solutions of thiourea and tetramethylthiourea. Conductometric detection showed that HO and a dimeric radical cation were produced. The dimeric radical cation is formed by addition of a primary radical to a molecule of thiourea. In basic solution, the dimeric radical cation decays rapidly to a dimeric radical anion, which is formed via neutralization of the cation and subsequent deprotonation of the neutral dimeric radical (Scheme 16). This was not observed in tetramethylurea. These dimeric radical cations of thiourea and tetramethylurea are strong oxidants and readily oxidize the superoxide radical, phenolate ion, and azide ion. 

Figure 12 compares the ESR spectra of the radical cations of silane dimers and their polymeric analogues. The observed spectra of polysilane radical cations are reproducible by using hyperfine parameters of the corresponding dimer radical cations and by assuming that the partial delocalization of the unpaired electron on only six Si atoms. 

The mechanism of dimer cleavage has been probed in model systems, including bifunctional ones in which a sensitizer (e.g., indole) is linked to the pyrimidine dimer. Work on a linked dimer (64), suggested that a dimer radical cation is a discrete (short-lived) minimum. ... 

When generated in zeolites, alkene or arene radical cations react with the parent molecules to form ti-dimer radical cations. For example, 2,3-dimethyl-l-butene and benzene formed 91 + and 92 +, respectively. The confinement and limited diffusion of the radical cation in the zeolite favor an interaction between a radical cation and a neutral parent in the same channel. 

Some radical cations with spin and charge on heteroatoms form a-dimer radical cations. For example, irradiation of acetophenone and aliphatic amines inside Na Y... 

PET converts phenylacetylene to ot-phenylnaphthalene a 1,4-bifunctional dimer radical cation (96 +) is the key intermediate 1,6-cyclization, followed by a hydrogen or hydride shift generates the final product. ... 

The mechanism of the cycloaddition appears to be concerted for various reagents however, for several cases, radical cation cycloaddition-cycloreversions have a stepwise component. For example, CIDNP effects observed during the PET induced dimerization of spiro[2.4]heptadiene (97) identify a dimer radical cation with spin density only on two carbons of the dienophile fragment this intermediate must be a doubly linked radical cation ( 99 + 282,283 pulsed laser experiment at high concentrations of 97 supports a second dimer radical cation at high... 

In addition to nucleophilic capture by alcohols, nonprotic nucleophiles also react with these intermediates. For example, the distonic dimer radical cation 96 + can be trapped by acetonitrile a hydride shift, followed by electron return, gave rise to the pyridine derivative 131. Similar acetonitrile adducts are formed in the electron-transfer photochemistry of terpenes such as ot- and (3-pinene ° or sabinene. ... 

Cationic species are also formed when sulfides (Meissner et al. 1967 Adams 1970 Bonifacic et al. 1975a Janata et al. 1980 Hiller et al. 1981 Davies et al. 1984 Ramakrishna Rao et al. 1984) or thioureas (Wang et al. 1999 Schuchmann et al. 2000) react with OH. Especially stable are the dimeric radical cations [reactions (48) - (50)]. In the case of thiourea, the high stability of the dimeric radical cation may contribute to the driving force which leads, in acid solution, to its forma-... 

Recently we have also reported, for the first time, the charge resonance (CR) band due to dimer radical cation formation as a broad absorption with a peak... 

Figure 23 Schematic representation of energy levels for monomer radical and dimer radical cations AH is the stabilization energy of dimer radical cations.

As shown in Fig. 25, the rise of absorption spectra at the visible region due to radical formation and at the near-IR region due to the CR band was observed in less than 1 ps upon a fs laser excitation at 400 nm. These results indicated that the dimer radical cations were formed immediately after the photoinduced electron transfer reaction. The CR band at 900 nm in NS+TPB- decayed single exponentially (t = 3.3 ps) [68], The transient absorption at 580 nm showed... 

The higher rate of the reverse electron transfer in the dimer radical cations than in the monomer radicals was explained by classical Marcus theory as follows [52,53]. The -AG° values for the reverse electron transfer from NS and DCS to TPB were estimated as 1.69 and 1.61 eV from the redox potentials, respectively. The reduction potential of the dimer radical cation should be less negative by 0.65 and 0.59 V than that of the radical monomer radicals due to the stabilization energy. The -AG° values for the reverse electron transfer from the dimer radical cation to TPB were thus estimated to be 1.04 and 1.02 eV for (NS+ -NS ) and (DCS+ -DCS ), respectively [68],... 

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