Thiyl radical optical absorption

The (US.. SU)" radical anion was generated via the one-electron reduction of USSU, initiated by hydrated electrons [analogous to reaction (37)], and is characterized by a broad optical absorption band with X = 450 nm, = 6000 M" cm" and a half-width of 1.0 eV. It exists in equilibrium with the conjugated thiyl radical (US ) and the corresponding thiolate ion (US") [reaction (41)]... 

Rate constants on the order of several 10 M s and 10 -10 s for the forward and back reactions, respectively, yield equilibrium constants of lO -lO" M [39-41], Although this indicates reasonable stability for the thioperoxyl radical it may nevertheless easily escape detection. In air saturated solutions ([O2] ca. 2.5 X lO M), for example, formation of thioperoxyl radicals occurs with r,/2 ca. 1 ps, which is of the same magnitude as the halflife of its decay. This is of particular significance when a distinction between the reactions of RS and RSOO is at stake, and wrong assignments may be made unless absolute kinetic and thermodynamic data are known and taken into account. Two of the still small number of systems for which accurate measurements are available deal with the respective thiyl and thioperoxyl radicals of mercaptoethanol and glutathione. At least in case of the mercaptoethanol derived thioperoxyl radical, investigations have been facilitated by a weak but distinct optical absorption of RSOO with near 550 nm (c ca. 200-400 M cm ) [37,42]. 

Table 3 Optical absorption spectra of some aliphatic thiyl radicals and their complexes.

Thiyl radicals easily add oxygen in glasses even at 77 K, forming thiyl peroxyl radicals, the species, first detected by their optical absorption in pulse radiolysis studies. These species are discussed in more detail along with other sulfoxyls in Chapter 8. Note here that thiol peroxyl radicals are fairly weakly bound species with a S-OO bond energy of only about 7-13 kcal mol" [99-103]. Thus any complexation of comparable strength, as with thiolate anions and neutral thiols... 

The list of sulfur-based peroxyls includes thiyl peroxyls, RSOO sulfuranyl peroxyls, R (R 0)3 S00 (/i=l, 2), and sulfonyls peroxyls, RSO2OO. In all cases the peroxyls are invariably formed by oxygen addition to the appropriate sulfur-based precursors. Thiyl peroxyl radicals were historically the first species in this group observed directly by their optical absorption in the visible in pulse radiolysis studies [7]. Since that time their actual structure has been under question [15, 87]. ESR results using O-labeling finally presented solid evidence for the true peroxyl structure of these species with two unequivalent oxygens [51 ... 

Oxygen addition to thiyl radicals is reversible at ambient temperatures [4, 9-11, 88] and appears to be diffusion controlled [4,88]. In glasses it takes place even at 77 K if the matrix allows oxygen to diffuse [13]. ESR and optical absorption spectra of some O-labelled RSOO radicals obtained in methanol glasses at 77 K are shown in Figure 1. A gas phase rate constant has been reported to be about 10 cm molecule" s within 216-258 K temperature range, although the equilibrium is shifted towards the reactants and this allowed this process to escape detection in the past [89]. 

Figure 1 ESR and optical absorption spectra of thiyl peroxyl radicals in a methanol glass at 77 K. Samples containing O2 and 0.2 M thiols were UV irradiated at 77 K and annealed at 103 K to form peroxyl radicals. The O hyperfine structure of RSOO is shown by the stick diagrams. Unspecified O features belong to carbon-based peroxyls from the matrix. Reproduced from Razskazovskii et ai [13] by permission of the Journal of Physical Chemistry.

Table 4 Spin density distributions and optical absorption maxima of thiyl peroxyl radicals RS0(2)0(d [13].

Flash photolysis/pulse radiolysis with optical detection are other techniques widely used in studies of sulfoxyl radicals. The major complication in the use of these techniques is the fairly weak absorptivity of sulfoxyl radicals combined with their low reactivity. Sulfonyl and thiyl peroxyl radicals are the only species... 

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