Aromatic molecules, cation radicals

Radiation techniques have been used extensively with non-aqueous systems as well. Much work has been done on aromatic molecule cations and anions and on electron transfer processes involving these species (see the review by Dorfman, 1970). These and other studies on radical ions, on excited states, and on charge-transfer complexes have been reviewed by Fendler and Fendler (1970). 

It may, however, be remembered that AlCl in nitromethane has been commonly used by many workers for oxidation of unsaturated and aromatic molecules to radical cations for the purpose of spectroscopic investigations.37 Even AlCl dissolved in CH2CI2 is a suitable oxidizing reagent to create the cation radicals of organic molecules whose first ionization potential is below 8 eV, i.e. of most of the polymers known to be converted to organic metals. 

Petrenko, A., K. Redding et al. (2005). The influence of the structure of the radical cation dimer pair of aromatic molecules on the principal values of a g-tensor DFT predictions. Chem. Phys. Lett. 406 327-331. 

Aromatic molecules can be polymerized catalytically on clean metal surfaces, or electrochemically to produce oriented polymer films. Initial adsorption of aromatic molecules occurs by electron donation from the aromatic molecule to the surface. This electron donation creates radical cations that can polymerize. Molecular orientation in the films depends on the stable bonding configuration of the radical cation. Thiophene, pyridines, and pyrrole all polymerize with the ring substantially perpendicular to the surface, whereas aniline polymerizes with the phenyl rings parallel to the surface. The catalytically... 

Two types of complex are formed on reaction of benzene with Cu montmorillonite. In the Type 1 species the benzene retains Its aromaticity and is considered to be edge bonded to the Cu(II), whereas in the Type 2 complex there is an absence of aromaticity (85,86). ESR spectra of the Type 2 complex consist of a narrow peak close to the free spin g-value and this result can be explained in terras of electron donation from the organic molecule to the Cu(II), to produce a complex of Cu(I) and an organic radical cation. Similar types of reaction occur with other aromatic molecules. However with phenol and alkyl-substituted benzenes only Type 1 complexes were observed (87), although both types of complex were seen on the adsorption of arene molecules on to Cu(II) montmorillonites (88) and anisole and some related aromatic ethers on to Cu(II) hectorite... 

In some cases, the ground state is not a singlet state, e.g. dioxygen, anion and cation radicals of aromatic molecules. 

Anodic oxidation in inert solvents is the most widespread method of cation-radical preparation, with the aim of investigating their stability and electron structure. However, saturated hydrocarbons cannot be oxidized in an accessible potential region. There is one exception for molecules with the weakened C—H bond, but this does not pertain to the cation-radical problem. Anodic oxidation of unsaturated hydrocarbons proceeds more easily. As usual, this oxidation is assumed to be a process including one-electron detachment from the n system with the cation-radical formation. This is the very first step of this oxidation. Certainly, the cation-radical formed is not inevitably stable. Under anodic reaction conditions, it can expel the second electron and give rise to a dication or lose a proton and form a neutral (free) radical. The latter can be either stable or complete its life at the expense of dimerization, fragmentation, etc. Nevertheless, electrochemical oxidation of aromatic hydrocarbons leads to cation-radicals, the nature of which is reliably established (Mann and Barnes 1970 Chapter 3). 

In comparison with hydrocarbons, aromatic amines easily transform into cation radicals. Structures of these cation radicals are well documented on the basis of their ESR spectra and MO calculations (see, e.g., Grampp et al. 2005). The stable cation radical of A/,A,A, A -tetramethyl-p-phenylenediamine (the so-called Wuerster s blue) was one of the first ion radicals that was studied by ESR spectroscopy (Weissmann et al. 1953). The use of this cation radical as a spin-containing unit for high-spin molecules has been reported (Ito et al. 1999). Chemical oxidation of N,N -bis [4-(dimethylamino)-phenyl-A/,A -dimethyl-l,3-phenylenediamine with thianthrenium perchlorate in -butyronitrile in the presence of trifluoroacetic acid at 78°C led to the formation of the dication diradical depicted in Scheme 3.58. 

Sometimes transformation of aromatic componnds into ion-radicals leads to stereochemically unusual forms. Octamethylnaphthalene is a nonplanar molecule twisted around the bond that is common for the two six-membered rings. The nitrosonium oxidation results in the formation of the cation-radical with the centrosymmetric flatten chairlike geometry (Rosokha and Kochi 2006). According to the authors, such a skeletal transformation improves the overall planarity of octamethylnaphthalene. For example, the mean deviation of the carbon atoms in the naphthalene core for the flatten chairlike cation-radical (0.007 nm) is less than half of the corresponding value for the neutral twisted parent (0.016 nm). Within this flatten carcass of the anion-radical, the spin density can be delocalized more effectively. 

For ESR studies, cation radicals of aromatic molecules have most generally been formed by dissolution of the parent compound in concentrated sulfuric acid.19 Neither this nor any of the several new chemical methods of generating these species in solution26-35 provides a particularly suitable medium for subsequent energetic chemical reduction. 

Conjugated conducting polymers consist of a backbone of resonance-stabilized aromatic molecules. Most frequently, the charged and typically planar oxidized form possesses a delocalized -electron band structure and is doped with counteranions (p-doping). The band gap (defined as the onset of the tt-tt transition) between the valence band and the conduction band is considered responsible for the intrinsic optical properties. Investigations of the mechanism have revealed that the charge transport is based on the formation of radical cations delocalized over several monomer units, called polarons [27]. 

The object of this work was to study the influence of pretreated, decationized NH4-zeolites on adsorbed A,iV-dimethylaniline molecules. Such influence is caused by, proton-donating and electron-deficient active sites in decationized zeolites. Interaction of an aromatic amine molecule (M) with the proton-donating site leads to the formation of the MH+ molecule ion interaction with the electron-deficient site results in the M+ cation radical. Stabilization of these states by adsorption leads to the... 

Interaction of the aromatic amine molecule (M) with the Si08/2 electron-deficient site must lead to the formation of the M+ cation radical and to a corresponding change in site as a result of electron capture ... 

Change in the substituent nature after transformation of neutral aromatics into the corresponding ion radicals may be used intuitively for preparation of some unusual derivatives. One may transform an organic molecule into its anion radical, change the substituent effect, perform the desired substitution, and, after that, take a surplus electron off by means of a soft oxidant and eventually obtain the desired unusual derivative in its stable form. Studies along this line are intriguing in the cases of both anion and cation radicals. 

In order to understand features of oxidative one-electron transfer, it is reasonable to compare average energies of formation between cation-radicals and anion-radicals. One-electron addition to a molecule is usually accompanied by energy decrease. The amount of energy reduced corresponds to molecule s electron affinity. For instance, one-electron reduction of aromatic hydrocarbons can result in the energy revenue from 10 to 100 kJ mol-1 (Baizer Lund 1983). If a molecule detaches one electron, energy absorption mostly takes place. The needed amount of energy consumed is determined by molecule s ionization potential. In particular, ionization potentials of aromatic hydrocarbons vary from 700 to 1,000 kJ-mol 1 (Baizer Lund 1983). 

Photoinduced electron-transfer reactions generate the radical ion species from the electron-donating molecule to the electron-accepting molecules. The radical cations of aromatic compounds are favorably attacked by nucleophiles [Eq. (5)]. On the contrary, the radical anions of aromatic compounds react with electrophiles [Eq. (6)] or carbon radical species generated from the radical cations [Eq. (7)]. In some cases, the coupling reactions between the radical cations and the radical anions directly take place [Eq. (8)] or the proton transfer from the radical cation to the radical anion followed by the radical coupling occurs as a major pathway. In this section, we will mainly deal with the intermolecular and intramolecular photoaddition to the aromatic rings via photoinduced electron transfer. 

Both the radical cation of VA and Mnm(S +) attack and degrade the lignin polymer LiP also has the ability to oxidize nonphenolic aromatic molecules, most likely by attacking a benzylic C—H bond. 

Oxidizing/Reducing Entities. Other reactions depend on oxidation/reduction processes. Among them is polymerization of various aromatic molecules. Such polymerizations are preceeded by formation of radical cations of the aromatic hydrocarbon. Cu-montmorillonites are capable of catalyzing such reactions (136). 

Very high acidity of a-protons in the radical cations of alkyl benzenes has been predicted by calculations and largely borne out in PET chemistry. Since it is expected that the C-H bond which is cleaved lies perpendicular to the plane of the aromatic molecule, the preferred conformation of the alkyl group should affect the efficiency of the cleavage. Thus as an example the reaction at the t-propyl and at the methyl group when DCN is irradiated in the presence of either one of the cymenes is different for each type of adduct formed, indicating that competition between the two possible modes of deprotonation depends on whether it occurs within the radical ion pair or from the free radical cation [63]. 

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