Sulfur-centered radical cations

Obviously, these sulfur-centered radical cations are good sinks for 02, and, for example, disulfide radical cations give rise to sulfoxides in a very fast reaction [reaction (53), k = 1.6 x 1010 dm3 mol1 s Bonifacic et al. 2000b]. 

One-electxon oxidation of organic sulfides (thioethers) leads primarily to a sulfur-centered radical cation, R2S, which is stabilized either by resonance (e.g. if R = Aryl) or interaction with a free electron pair provided by a second sulfur atom (typically in the case of aliphatic sulfides). The latter stabilization occurs inter- or intm-molecularly with the former depending on the sulfide concentration, while the intramolecular process requires a geometry of the two interacting p orbitals which is favorable for overlap. This concept has been verified in numerous experiments and supported by theoretical calculations. ... 

Sulfur-centered radical cations derived from aromatic thioethers (Ar-S-Ar) have been investigated much less extensively. An important feature of one-electron oxidation of aromatic thioethers is the lack of dimeric radical sulfur radical cations (ArS.. S-Ar) because of the spin delocalization onto the aromatic ring. Oxidation of thioanisole (Ar-S-CHj) by OH radicals was studied using pulse radiolysis. At neutral pH, OH addition led to the prompt formation of monomeric sulfur radical cations and hydroxycyclohexadienyl radicals (see Scheme 8). 

Sensitized photooxidation of sulfur-containing amino acids in neutral aqueous solution occurs via electron transfer from the sulfur to the triplet state of the sensitizer (the mechanism can be adapted to other dye-sensitized photoinitiated polymerization). Electron transfer quenching in the system is followed mainly by diffusion of the sulfur-centered radical cation and benzophenone radical anion. Laser... 

At pH above pKu2, where the amino group is deprotonated, these substrates contain two possible donor sites, sulfur and nitrogen. From the polarization patterns, it could be concluded that 48 quenches the sensitizer triplet by electron transfer from sulfur only. The CIDNP effects arise in two successive radical pairs because the sulfur-centered radical cations 50 +... 

All the sulfur-centered radical cations mentioned above exhibit moderate to strong optical absorptions allowing convenient detection and study of their properties by pulse radiolysis. In case of (R2S)2 and (RSSR) " the main absorption bands are located in the near UV and visible part of the spectrum. Since they are related to the special features of the odd-electron bonds it is again appropriate to present and discuss further details later. 

The basic feature of the sulfiir-sulfur bond in the dimer radical cation (R2S)2 " introduced above is that it contains three electrons, namely, two bonding a-electrons and one antibonding a -electron. Formally, it can be viewed as resulting from an interaction of a singly occupied p-orbital of the sulfur-centered radical cation, R2S ", and the p-lone pair of a second non-oxidized sulfur.56.85 jg depicted in eq. 40a and associated orbital... 

SULFUR-CENTERED RADICAL CATIONS AND ODD-ELECTRON BONDED INTERMEDIATES... 

The two headline topics are closely connected and it, therefore, makes sense to deal with them together. This does not mean that all sulfur-centered radical cations are odd-electron bonded species and vice versa. In fact, there are even several kinds of odd-electron species. The most frequent ones belong to the 2-center/3-electron (2(t/1(t ) radical cation category, but examples will also be discussed in which such transients carry a negative or no charge, or not only three but five electrons participate in the bonding. 

In principle, any one-electron oxidant with a high enough redox potential (> 1.5 V) [82], as well as electrochemical [83, 84] or photochemical methods [85, 86] may be used to generate such sulfur-centered radical cations directly. Their formation (monomer and dimer species) is not limited to aqueous solutions but seems possible in practically any environment, including hydrocarbon solutions [87], neat dimethylsulfide [88], or in the gas phase, including mass spectrometry conditions [89-93]. 

The added hydroxyl function may, of course, also be removed by externally available protons, for example, in highly acidic solutions. In this case the sulfur-centered radical cation will, however, not couple with the nitrogen but associate with the sulfur of an unoxidized molecule to form the dimeric (>S Sradical cation, reaction (69b). 

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