Carbon-centered radicals unpaired electron

Both the oxygen and sulfur atoms have two lone pairs while the C/ carbon has ar unpaired electron, and in both cases the double bond shifts from the two carbor atoms to the carbon and the substituent. In acetyl radical, the electron density i centered primarily on the C2 carbon, and the spin density is drawn toward the lattei more than toward the former. In contrast, the density is more balanced between thf two terminal heavy atoms with the sulfur substituent (similar to that in allyl radical with a slight bias toward the sulfur atom. These trends can be easily related to th< varying electronegativity of the heavy atom in the substituent. 

Numerous reports published in recent years have focused on carbon-centered radicals derived from compounds with selected substitution patterns such as alkanes [40,43,47], halogenated alkanes [43,48,49,51-57], alkenes [19], benzene derivatives [43,47], ethers [51,58], aldehydes [48], amines [10,59], amino acids [23,60-67] etc. Particularly significant advances have been made in the theoretical treatment of radicals occurring in polymer chemistry and biological chemistry. The stabilization of radicals in all of these compounds is due to the interaction of the molecular orbital carrying the unpaired electron with energetically and spatially adjacent molecular orbitals, and four typical scenarios appear to cover all known cases [20]. 

Reactivity of carbon-centered radicals may remain high even in crystalline state of the matrix when the access of oxygen towards unpaired electrons is not hindered by the sterical arrangement of surrounding molecules. This is, e.g., the case of peroxyl radicals formed during irradiation of cholesterol [60]. Oxygen reacts with alkyl radicals derived from cholesterol already at 125 K which is far below the melting point of the matrix (423 K). Two peroxyl radicals of cholesterol were observed (Scheme). 

In most cases, carbon-centered radicals carry the unpaired electron in p(7t)-type orbitals. Since C isotopes are present at a very low percentage (natural abundance, 1.11%), the spin distribution is monitored by adjacent H nuclei. The spin from the p-type orbital of the C atom is transferred to the adjacent H atom (H ) via n-spin polarization (Fig. 7.2a) however, spin transfer to more distant protons follows the model of a hyperconjugation mechanism illustrated in Fig. 7.2b. The closer the Cp-Hp is oriented toward the z-axis of the C p orbital, (0 = 0°) the bigger becomes the Hp hfc. When 0 is equal to 90°, the Hp hfc reaches its minimum value. This behavior is described by an empirical formula ... 

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. 

The ff-t5Tpe ethoxycarbonyl radical is on the contrary less nucleophilic than the acetyl radical (Table 29) in this Ccise the unpaired electron occupies a hybrid orbital and the incipient positive charge in the transition state cannot be stabilized by the lone-pair electron of the alkoxy group, as with the alkoxyalkyl radical, so that only the inductive effect is working and a clean reduction of nucleophilicity is observed. The remarkable fact is therefore that the same substituent, an a-alkoxy group, produces opposite polar effects depending on the electronic configuration of the carbon-centered radical. 

A carbon-centered radical is a structure with a formal charge of 0 and one unpaired electron on a carbon atom. Examples include the methyl radical (1), the vinyl radical (2), the phenyl radical (3), and the triphenylmethyl radical (4). Radicals are often called radicals, a term that arose from early nomenclature systems in which a radical was a substituent group that was preserved as a tmit through a chemical transformation. Thus, the CH3 group as a substituent was known as the methyl "radical," so a neutral CH3 group... 

The propagating spedes in most free-radical polymerization reactions is a n-type carbon-centered radical in which the unpaired electron is located in a p-lype orbital and, for a monomer of the general form CH2=CXY, the sp -hybridized radical center is substituted with the monomer substituents X and Y, and the remaining polymer chain (see Figure 1). The chemistry of free-radical polymerization is profoundly shaped by the effeas of these substituents on the stability of the propagating radical, and the broader relationships between its stability and its reactivity in the various possible reactions and side reactions that occur. 

The above radical stability schemes all measure the stabilization energy of a radical from its contribution to various bond energies an alternative approach is to use measurements of the extent of delocalization of the unpaired electron. Since n-type carbon-centered radicals are stabilized by substituents that delocalize the unpaired electron, the more delocalized the unpaired electron is, the more stable the radical is likely to be. This allows one to focus solely on the radical, thereby avoiding complications from substituent effects on the... 

Carbon-centered organic radicals are highly reactive trivalent species with only one nonbonding electron. While most known radicals have their unpaired electron in a pure p- or a delocalized Ji-orbital, there are also examples of radicals centered in s/t" hybrid o-orbitals, such as the well known phenyl and cyclopropyl radicals. The first radical reported in the literature is credited to Gomberg s landmark paper in 1900 when he postulated the formation of triphenylmethyl radical 36, also known as tri-fyj 99,100 jj-jjyj j-adical is an example of a persistent radical that exists in equilibrium... 

Any molecular entity possessing an unpaired electron. The modifier unpaired is preferred over free in this context. The term free radical is to be restricted to those radicals which do not form parts of radical pairs. Further distinctions are often made, either by the nature of the central atom having the unpaired electron (or atom of highest electron spin density) such as a carbon radical (e.g., -CHs) or whether the unpaired electron is in an orbital having more s character (thus, radical molecular entity in a manuscript, the structure should always be written with a superscript dot or, preferably, a center-spaced bullet (e.g., -OH, -CHs, CF). 

For closed-shell molecules (in which all electrons are paired), the spin density is zero everywhere. For open-shell molecules (in which one or more electrons are unpaired), the spin density indicates the distribution of unpaired electrons. Spin density is an obvious indicator of reactivity of radicals (in which there is a single unpaired electron). Bonds will be made to centers for which the spin density is greatest. For example, the spin density isosurface for allyl radical suggests that reaction will occur on one of the terminal carbons and not on the central carbon. 

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