Carbon-centered radicals electronic properties

The mechanism of melatonin s interaction with reactive species probably involves donation of an electron to form the melatoninyl cation radical or through a radical addition at the site C3. Other possibilities include hydrogen donation from the nitrogen atom or substitution at position C2, C4, and C7 and nitrosation [169]. The mechanisms by which melatonin protects against LP most likely involve direct or indirect antioxidant and free-radical scavenging activities of this indoleamine [169,171]. 2-Phenyl indole derivatives have redox properties because of the presence of an electron-rich aromatic ring system that allows the indoleamine to easily function as an electron donor. For these derivatives, the possible antioxidant mechanism might be most probably toward carbon-centered radicals described by Antosiewicz et al. [172]. 

Carbon-centered radicals play a significant role in a number of processes of technological and biological relevance, but as most of the radicals are unstable and very reactive species, difficult to study by experimental techniques. Many available experimental studies focused on the physicochemical properties of radicals come from the spectroscopic techniques, not only electronic spin resonance (ESR) but also IR/Raman and UV-vis, often obtained in matrices at low temperature. Unfortunately, in most cases, experimental results are extremely difficult to interpret, even in terms of appropriate identification of molecular species. For example, the entire UV-vis... 

Generalized to any type of carbon-centered radical, this approach has led to use of C—H bond dissociation energies for estimating the unpaired electron delocalization energies of these species. As shown in Tables XXXIII, XXXIV, and XXXV, bond dissociation energies which, except for a constant, are equal to heats of reaction do not provide satisfactory resonance (or stabilization) energies of free radicals. Indeed, as stated before, a heat of reaction can never be used for determining any property of one of the species involved in that reaction. 

V. Barone, M. Biczysko, P. Cimino, Interplay of stereo electronic vibrational and enviromnental effects in mning physico-chemical properties of carbon centered radicals, in Carbon-Centered Free Radicals and Radical Cations, M. D. E. Forbes, Ed., Willey, Hoboken, NJ, 2010, pp. 105-139. 

Traditionally, electron transfer processes in solution and at surfaces have been classified into outer-sphere and inner-sphere mechanisms (1). However, the experimental basis for the quantitative distinction between these mechanisms is not completely clear, especially when electron transfer is not accompanied by either atom or ligand transfer (i.e., the bridged activated complex). We wish to describe how the advantage of using organometals and alkyl radicals as electron donors accrues from the wide structural variations in their donor abilities and steric properties which can be achieved as a result of branching the alkyl moiety at either the a- or g-carbon centers. 

Radical cations are strongly oxidizing intermediates, but also after deprotonation at a heteroatom (in the present systems at nitrogen) some of this oxidizing property remains. Thus a common feature of these intermediates is that they are readily reduced by good electron donors. Since the heteroatom-centered radicals and the radical cations are always in equilibrium, it is, at least in principle, possible that such intermediates react with water at another site (canonical mesomelic form), that is at carbon. This reaction leads to OH-adduct radicals. Although deprotonation at a heteroatom is usually faster (but also reversible) than deprotonation at carbon, the latter reaction is typically "irreversible". This also holds for a deprotonation at methyl (in Thy). 

Aminocyclopropane derivatives are known to possess enzyme inhibitory properties.In particular, aminocyclopropanes are potent inhibitors of cytochrome P-450 mono-oxygenases. A carbon-centered free radical, generated by one-electron oxidation of aminocyclopropane by P-450 to form a nitrogen-centered radical cation and subsequent ring opening, may play an important role in the destruction of the enzyme. 

The chemical properties of these free radicals were studied in small molecules such as dithiothreitol, lipoamide and glutathione etc... (For reviews see 7, 132). The thiyl radical is oxidizing (table 5). However, it is in tautomeric equilibrium with a carbon-centered free radical (49, 133) formed through base-catalyzed intramolecular rearrangement. This latter form is a reductant (49) and its fate is unknown. Thiyl radicals are also formed in proteins. They are somewhat uneasy to visualize because of their low extinction coefficient (table 3). In addition when they are formed by one-electron oxidation, tyrosinyl radicals are also created and thus thiyl radicals are hidden by their strong absorption (134, 135). However they may be observed when formed by reduction (37). 

Photopolymerization systems, like thermally initiated systems, contain initiator, monomer, and other additives that impart desired properties (color, strength, flexibility, etc) (6). The reaction is initiated by active centers that are produced when light is absorbed by the photoinitiator. One important class of active centers includes free-radical species, which possess an impaired electron (5,7). The highly reactive free-radical active centers attack carbon-carbon double bonds in imsaturated monomers to form pol5nner chains. Although the kinetic treatment of photopoljnner systems is similar to that in thermal systems, significant differences arise in the description of the initiation step, which in turn affect the... 

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