Peroxyl spin densities

The spin density is concentrated on the last oxygen atom in the peroxyl radical R(20)(10 ) (about 80%). However, about 20% of the spin density is concentrated on the (20) atom also (see Table 2.6). This proves the existence of rather strong interaction between electronic clouds of two oxygen atoms and agrees with the length of O—O bond in the peroxyl radical (see Table 2.5). 

The similar reactivity of peroxyl with (CF3)2NO radicals towards a large variety of substrates has been observed previously and attributed to rather similar thermochemistries and spin density distributions for these two radicals [30]. The Arrhenius parameters for the reaction of the persistent (CF3)2NO radical with n-BusSiH determined by kinetic EPR spectroscopy are logyl/M s = 5.5 and Ea = 32.2kJ/mol, which corresponds to k = 4.3 s- at 73 °C [31]. 

Geometries, Spin Densities, Oxidative Power and pKa Values of Peroxyl Radicals 165... 

Raiti MJ, Sevilla MD (1999) Density functional theory investigation of the electronic and spin density distribution in peroxyl radicals. J Phys Chem A 103 1619-1626... 

In this review the generic term sulfoxyl radical is used to define species with oxygens directly bonded to sulfur and carrying a substantial spin density in the sulfur-oxygen moiety. Species that fit this definition are sulfinyl, RSO sulfonyl, RSO2 thiyl peroxyl, RSOO sulfuranyl peroxyl, (RO) R 3 SOO sulfonyl peroxyl, RSO2OO and various kinds of sulfuranyl radicals, (RO) R3 S and SO species. Sulfuranyl and SO/ radicals will be mentioned only briefly in this chapter as they are reviewed separately in this book (see Chapter 9 by Margaretha and Chapter 3 by Huie and Sieck, respectively). 

All experimental information concerning spin density distributions in sulfur-based peroxyls was obtained using O labeling and ESR spectroscopy in solids. Some selected data are presented in Table 3. 

These data indicate that the sulfur-based peroxyls in general have a lot in common with carbon-based peroxyl radicals. They display essentially the same extent and character of g anisotropy as carbon-based peroxyl radicals with some minor variations. The g iax values are slightly lower for sulfuranyl peroxyls and substantially higher for sulfonyl peroxyls compared to those of carbon-based peroxyls. Due to this feature the spectra of sulfonyl and carbon-based peroxyls can be separated in solids even without labeling [72]. There is also a pronounced trend in the couplings reflecting variations in spin density distribution within the peroxyl group. 

For peroxyl radicals the parallel O hyperfine coupling from each oxygen atom has been considered to be proportional to the n spin density on that atom [98]. It has been established experimentally that the sum of oxygen couplings in peroxyl radicals, including 02, numerous carbon-based peroxyls sulfuranyl and sulfonyl peroxyls, is close to 154 G. This value is taken to be the Ay splitting... 

There are three distinct effects observable in Table 4. The first is a change in spin density distribution in the same radical on changing the matrix. This trend is best observed in the case of /-BuSOO where the solvent influence is most pronounced. The solvent property responsible for this effect was suggested to be an ability to solvate electron-deficient centers [13]. Theoretical calculations (mentioned previously) suggested the sulfur atom carries a substantial positive charge in thiyl peroxyls. 

A partial charge transfer from the fairly oxidizable divalent sulfur to the fairly oxidative peroxyl oxygens has been proposed to explain the difference between thiyl and other peroxyls [13, 98]. The two extremes of charge transfer in thiyl peroxyls can be represented by the bipolar structures, RS -0-0 and RS -02. The first structure explains the partial spin density on the sulfur atom. 

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

Oxygen-17 hyperfine couplings. Total spin density on peroxyl oxygens. The remainder is presumed to be located on the sulfur. Two RSOO species with different O couplings are observed. 

The second is a complex of sulfenium cation and superoxide and explains the near equivalence of the spin density on the two peroxyl oxygens in thiyl peroxyls. Both valence structures display an exposed electron-deficient sulfur that makes it susceptible to nucleophilic solvents. Complexing with a lone pair donor is expected to stabilize the charge-separated state increasing its contribution to the ground state. A partial radical-cationic character of the sulfur could also make three-electron bonding with some solvents contribute to the charge separation. 

An attempt has been made to mimic the solvent influence on spin density distribution in thiyl peroxyl radicals at the UHF 6-3IG theoretical level. It should be noted that SCF calculations typically fail to reproduce experimental spin density distributions even in ceirbon-based peroxyls, placing more spin density on the terminal oxygen and less on the inner one. Thus, the objective of this analysis at this level could be a comparison of tendencies, not exact numbers. Some selected structures (10)-(12) considered in that study are shown [13]. 

The spin density distributions in sulfonyl peroxyls match those found for a-fluorinated alkyl peroxyls. Thus they fit a general trend displayed by peroxyls with stronger electron withdrawing substituents which show larger terminal spin densities. A correlation has been shown to exit between those spin densities and the reactivity of carbon-based peroxyls [98]. If this holds for sulfonyl peroxyls as well, they would be expected to be among the most reactive peroxyls known. 

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