**Author details**

280 Nitroxides – Theory, Experiment and Applications

2007; Offer & Samuni, 2002).

**5. Conclusions** 

A fundamental question about the detailed mechanism of the reaction (5.1) remains open. By analogy with the oxidation of 1,2-substituted ethylenes and 1,4-substituted butadienes (Mogilevich & Pliss, 1990) we can assume that hydroxylamine's formation is facilitated by

~OO–CH2–CH(C6H5)–OO• + >NO• ~OO–CH2=CH–C6H5 + >NOH + O2 Peroxide bridge is an important structural unit of ~MO2• radical. It alters the reaction center's electronic characteristics and increases the electrostatic term's contribution to the transition state's total energy (Denisov, 1996; Denisov & Afanas'ev, 2005; Mogilevich & Pliss, 1990). Probable reason of this effect is the difference in the triplet repulsion, which is close to zero in transition state of disproportionation reaction of MO2• with >NO• and is sufficiently large for the reaction of >NO• with nonconjugated C–H bond of hydrocarbon (Denisov, 1996). The latter probably explains the fact that aliphatic nitroxide radicals inhibit the

The results obtained in the present study draws attention to the results gained for biological systems where it is assumed that reaction of aliphatic >NO• with peroxide radicals proceeds via >NOOOR adduct formation decomposing to corresponding oxoammonium cations (Barton et al., 1998; Goldstein & Samuni, 2007; Offer & Samuni, 2002). The probability of such intermediate's existence is also considered in quantum-chemical analysis (Hodgson & Coote, 2010; Stipa, 2001). Further regeneration of nitroxide radicals may be due to reaction of oxoammonium cations with common biological reducing agents (Goldstein & Samuni,

Direct reaction MO2• + >NO• MOOON< that results to stable trioxide's formation is seems quite doubtful for aliphatic >NO• in organic phase at moderate temperatures ( 373 K). First, it's easy to reject on the base of kinetic reasons cause in this case the kinetics of >NO• consumption and stoichiometry of chain termination would have a different nature than those observed in numerous studies (Browlie & Ingold, 1967; Kovtun et al., 1974; Pliss et al., 2010a, 2010b, 2012; Pliss & Aleksandrov, 1977). Second, it's easy to refute by direct quantum-chemical calculations (DFT B3LYP/6-31G\*, Table 7). It's easy to see that peroxide

Radical H• HO• HOO• •CH3 CH3O• CH3OO•

Energy –243.4 –28.3 48.1 –150.6 11.5 60.3

Thus, we must conclude that reaction of nitroxide with peroxide radicals plays an important role during styrene's oxidation in presence of aliphatic stable >NO•. This reaction proceeds

probably as disproportionation and results to a partial >NO• regeneration.

conjugation of β-C–H bond with peroxide bridge of styrene's polyperoxide radical:

hydrocarbon's oxidation via reaction with alkyl radicals only.

radicals' addition to >NO• is thermodynamically unfavorable.

**Table 7.** Energy of some radicals' addition to >NO• (I), kJ/mol

Eugene M. Pliss and Alexander I. Rusakov *P.G. Demidov Yaroslavl State University, Russia* 

Ivan V. Tikhonov *Yaroslavl Branch of the Institute of Physics and Technology, Russian Academy of Sciences, Russia* 
