Solvent radical cation

Phenol radical cations exist only in strong acidic solutions (pKa -1) [1, 2]. However, in non-polar media phenol radical cations with lifetimes up to some hundred nanoseconds were obtained by pulse radiolysis [3], The free electron transfer from phenols (ArOH) to primary parent solvent radical cations (RX +) (1) resulted in the parallel and synchroneous generation of phenol radical cations as well as phenoxyl radicals in equal amounts, caused by an extremely rapid electron jump in the time scale of molecule oscillations since the rotation of the hydroxyl groups around the C - OH is strongly connected with pulsations in the electron distribution of the highest molecular orbitals [4-6]. 

Both the styrene monomer and the neutral dimer can trap a migrating positive hole or positive charge from solvent radical-cations (solventt) or related cationic species, which leads to the formation of radical cations, dimer cations, and bonded dimer cations. 

The positive charge of solvent radical-cations transfers to solute molecules in halogenated hydrocarbons such as chloroform and dichloroethane. However, only few studies have been made on the radical cations of polymers in solution. Tanaka et al. observed the dimer cation of the biphenyl group or the pyrenyl group of the polymers in the pulse radiolysis of PVB and PVP in 1,2-dichloroethane [49]. The absence of the monomeric cation is due to the rapid intramolecular dimerization of the radical cations of the side groups of the polymers. Irie et al. observed two kinds of intramolecular dimer cations in the... 

Parent radical cations derived from alkanes and alkyl chlorides can be directly observed in the nanosecond time domain by time-resolved spectroscopy such as laser flash photolysis and electron pulse radiolysis. Especially the latter one enables the direct ionization of the solvents independently on the optical properties of the sample and a well-defined electron transfer regime according to Eq. (2) or (3). Representative examples of the radiolyfic generation of solvent radical cations are given in Eqs. (4) and (5a) for the cases of 1-chlorobutane and -decane. ... 

The unrestricted and free electron transfer (FET) from donor molecules to solvent radical cations of alkanes and alkyl chlorides has been studied by electron pulse radiolysis in the nanosecond time range. In the presence of arenes with hetero-atom-centered substituents, such as phenols, aromatic amines, benzylsilanes, and aromatic sulfides as electron donors, this electron transfer leads to the practically simultaneous formation of two distinguishable products, namely donor radical cations and fragment radicals, in comparable amounts. 

Mehnert R, Brede O, Naumarm W. (1982) Charge transfer from solvent radical cations to solutes in pulse-irradiated liquid -butylchloride. Ber Bunsen es Phys Chem 86 525-529. 

Brede O, Ganapathi MR, Naumov S, Naumann W, Hermann R. (2001) Localized electron transfer in nonpolar solution Reaction of phenols and thio-phenols with free solvent radical cations. /Phys Chem A 105 3757-3764. 

Free electron transfer from xanthenyl and fluorenylsilanes (Me-3 or Ph-3) to parent solvent radical cations Effect of molecule dynamics. J Phys Chem A 109 11679-11686. 

One-electron oxidants can be generated by pulse radiolysis of acetone containing appropriate solutes. For example, it has been shown that solutions of KSCN and KBr produce (SCN)2 and Br2 , respectively [19], and, similarly, NO3 and Cl2 are formed in solutions of LiN03 and LiCl [20], These oxidants are formed in significant yields, presumably by reaction of the corresponding anion with the solvent radical cation (CH3)2CO +, and are sufficiently long-lived to permit their electron-transfer reactions to be studied in this solvent. 

With the objective of oxidizing the fullerene core, radiolysis of any chlorinated hydrocarbon solvent provides the means of forming strongly oxidizing radical species [71]. For example, the radiation-induced ionization of dichloroethane (DCE) yields the short-lived and highly reactive solvent radical cation. In general, the electron affinity of [DCE] + is sufficient to initiate one-electron oxidation of the fullerene moiety (Eq. 6) [72-76]. 

The very rapid oxidation of phenols by solvent radical cations can be expected to yield phenol radical cations as the first products. These species are short-lived, except in highly acidic solutions, and were not observed in the microsecond pnlse radiolysis experiments described above. They were detected, however, in frozen matrices and with nanosecond pulse radiolysis Gamma irradiation of phenols in w-butyl chloride or in l,l,2-trichloro-l,2,2-trifluoroethane (Freon 113) at 77 K produced phenol radical cations, which were detected by their optical absorption and ESR spectra . Annealing to 133 K resulted in deprotonation of the radical cations to yield phenoxyl radicals. Pulse radiolysis of p-methoxyphenol and its 2,6-di-fert-butyl derivative in w-butyl chloride at room temperature produced both the phenol radical cations and the phenoxyl radicals. The phenol radical cations were formed very rapidly k = 1.5 x 10 ° M s ) and decayed in a first-order process k = 2.2 x 10 s ) to yield the phenoxyl radicals. The phenoxyl radicals were partially formed in this slower process and partially in a fast process. The fast process of phenoxyl formation probably involves proton transfer to the solvent along with the electron transfer. When the p-methoxy group was replaced with alkyl or H, the stability of the phenol radical cation was lower and the species observed at short times were more predominantly phenoxyl radicals. 

Pulse radiolysis of N20-saturated methylcyclohexane (MCH) gives rise to the formation of the solvent radical cation, MCI-T, but in argon-saturated MCH, the olefmic fragment cation (methytcyclohexene) is obtained [3b],... 

Therefore, it appears that only 50-65% of the ionizations produce the electron-hole pairs that involve solvent radical cations. 

Oxidation The radiation-chemically induced ionization of chlorinated hydrocarbons, i.e., dichloroethane (DCE) leads to the initial generation of the corresponding solvent radical cation, [DCE] ". The electron affinity of the latter is sufficient to oxidize the fullerene moiety ([60]fullerene E1/2 = +1.26 versus Fc / Fc ). Pulse radiolytic experiments with [60]fullerene in nitrogen-saturated or aerated DCM solutions yielded a doublet with maxima at 960 and 980 nm (Figure 1) (12-18). This fingerprint is identical to that detected in photolytic oxidation experiments and that computed in CNDO/S calculations. Rate constants for the [60]fullerene oxidation are typically very fast with estimated values IC7 > 2 x 10 ° M s. The 7t-radical cation is short-lived and decays via a concentration-dependent bimolecular dimerization reaction with a ground state molecule (kg = 6 x lO M s ) (13). 

The observation that pulse radiolysis of NaO-saturated methylcyclohexane gives the solvent radical cation but that the argon-saturated solution gives the olefinic methyl-cyclohexene radical cation is attributed to the formation of a common excited-state precursor which then either fragments (Ar) or is quenched (NaO). Rate constants for the various processes have been measured. 

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