Electrons in radicals

The electron spin resonance spectrum of a free radical or coordination complex with one unpaired electron is the simplest of all forms of spectroscopy. The degeneracy of the electron spin states characterized by the quantum number, ms = 1/2, is lifted by the application of a magnetic field, and transitions between the spin levels are induced by radiation of the appropriate frequency (Figure 1.1). If unpaired electrons in radicals were indistinguishable from free electrons, the only information content of an ESR spectrum would be the integrated intensity, proportional to the radical concentration. Fortunately, an unpaired electron interacts with its environment, and the details of ESR spectra depend on the nature of those interactions. The arrow in Figure 1.1 shows the transitions induced by 0.315 cm-1 radiation. 

The presence of an unpaired electron in radical cations has significant consequences for symmetric compounds. This becomes clear if one considers the transformation of an open shell symmetric compound into another compound of a different symmetry. In such cases, the symmetry of the singly occupied molecular orbital (SOMO) determines the overall electronic states of the reactant and the product. If the two electronic states do not correlate, i.e., do not share a common symmetry element, a symmetry-preserving pathway from reactant to product is not possible. Any adiabatic reaction leading from the reactant to the product therefore has to involve the loss of symmetry. This problem obviously does not occur for the case of closed-shell molecules, where all orbitals are doubly occupied, leading to a common electronic Ai state for all molecules. 

The second technique which both confirms some of our deductions and provides useful quantitative data for frontier orbital analysis is electron spin resonance spectroscopy9,10 (ESR). This technique detects the odd electron in radicals the interaction of the spin of the electron with the magnetic nuclei (1H, 13C, etc.) gives rise to splitting of the resonance signal, and the degree of splitting is proportional to the electron population at the nucleus. Since we already know that the coefficients of the atomic orbitals, c, are directly related... 

The delocalization of the unpaired electron in radicals by substituents provides stabilization, and the resulting substituent effects have been quantitatively assessed in terms of the so-called radical stabilization energy (RSE) [42-44,48]. For the cumyl radicals 14, the RSE values may be estimated theoretically from the rotational barrier of the terminal isopropylidene... 

Fig. 4. Chain reaction of lipid peroxidation. An oxidant removes an electron from a PUFA (step 1) to form a lipid radical. Molecular rearrangement causes formation of a reactive conjugated diene (step 2). This can react with active singlet molecular oxygen ( 02), which is in an excited state rather than the ground state to form a peroxyl radical (step 3). Also, transition metals can react with oxygen to produce potent metal-containing oxidants that may allow simultaneous binding or bridging of a biomolecule and oxygen (B6, K4, W5). The peroxyl radical can be detoxified by an antioxidant to a lipid peroxide (step 4) or the peroxyl radical can act as an oxidant to remove an electron from another PUFA (step 5), effecting a chain reaction of autooxidation. PUFA, polyunsaturated fatty acid (R5). Dot indicates unpaired electron in radical forms.

Early theoretical works in the twentieth century are Pauling and Wheland 1933 Hiickel 1934 Ingold 1934. Intriguingly, the latter explicitly discusses chemical vs. physical explanations of the radical phenomenon. The first account of unpaired electrons in radicals and their paramagnetic properties was published by Gilbert Lewis (1923). 

Polyions should be distinguished from macroions, which carry only one or two ionizable groups per chain. The chain carrier resulting in a cationic polymerization is, for example, a macromolecule with a positive charge at the growing end of the chain, and as such is a macrocation. Correspondingly, a macroradical is a macromolecule with a free electron. In radical graft reactions, it is possible in some circumstances to have polyradicals with several radical sites per molecule. 

In the simple Hiickel MO (HMO) method, the odd electron in radicals of odd alternant hydrocarbons such as methyl, allyl, benzyl and benzhydryl, pertains to a nonbonding MO hence, these radicals should have the same ionization potentials. Actually, the ionization potentials vary over more than a 2-e.v. range (Table I). The... 

An atom or a molecule with the total spin of the electrons S = 1 is said to be in a triplet state. The multiplicity of such a state is (2.S +1)=3. Triplet systems occur in both excited and ground state molecules, in some compounds containing transition metal ions, in radical pair systems, and in some defects in solids. 

Sekiguchi S, Kobori Y, Akiyama K and Tero-Kubota S 1998 Marcus free energy dependence of the sign of exchange interactions in radical ion pairs generated by photoinduced electron transfer reactions J. Am. Chem. Soc. 120 1325-6... 

Goudsmit G-H, Paul H and Shushin A I 1993 Electron spin polarization in radical-triplet pairs. Size and dependence on diffusion J. Phys. Chem. 97 13 243-9... 

FIGURE 4 19 Bonding in methyl radical (a) If the structure of the CH3 radical IS planar then carbon is sp hybridized with an unpaired electron in 2p orbital (b) If CH3 IS pyramidal then car bon IS sp hybridized with an electron in sp orbital Model (a) IS more consistent with experimental observa tions... 

Not all the properties of alkenes are revealed by focusing exclusively on the func tional group behavior of the double bond A double bond can affect the proper ties of a second functional unit to which it is directly attached It can be a sub stituent for example on a positively charged carbon in an allylic carbocation, or on a carbon that bears an unpaired electron in an allylic free radical, or it can be a substituent on a second double bond in a conjugated diene... 

The unpaired electron in benzyl radical is shared by the benzylic carbon and by the nng carbons that are ortho and para to it as shown by the spin density surface in Figure 119 Delocalization of the unpaired electron from the benzylic carbon to the ortho and para positions can be explained on the basis of resonance contributions from the fol lowing structures... 

The unpaired electron in benzyl radical is shared by the benzylic... 

Free radical (Section 4 16) Neutral species in which one of the electrons in the valence shell of carbon is unpaired An ex ample is methyl radical CH3... 

A UHF wave function may also be a necessary description when the effects of spin polarization are required. As discussed in Differences Between INDO and UNDO, a Restricted Hartree-Fock description will not properly describe a situation such as the methyl radical. The unpaired electron in this molecule occupies a p-orbital with a node in the plane of the molecule. When an RHF description is used (all the s orbitals have paired electrons), then no spin density exists anywhere in the s system. With a UHF description, however, the spin-up electron in the p-orbital interacts differently with spin-up and spin-down electrons in the s system and the s-orbitals become spatially separate for spin-up and spin-down electrons with resultant spin density in the s system. 

Free Radicals. In the formula of a polyatomic radical an unpaired electron(s) is(are) indicated by a dot placed as a right superscript to the parentheses (or square bracket for coordination compounds). In radical ions the dot precedes the charge. In structural formulas, the dot may be placed to indicate the location of the unpaired electron(s). 

In principle, this degradation can continue until the residual radical contains only hydrogen or methyl groups attached to the carbon with the odd electron. Those radicals which stiU contain a carbon—carbon bond can form an olefin via reaction 23 (or sequence 2, 24). Methyl radicals are a special case with limited options. 

Weak to moderate chemiluminescence has been reported from a large number of other Hquid-phase oxidation reactions (1,128,136). The Hst includes reactions of carbenes with oxygen (137), phenanthrene quinone with oxygen in alkaline ethanol (138), coumarin derivatives with hydrogen peroxide in acetic acid (139), nitriles with alkaline hydrogen peroxide (140), and reactions that produce electron-accepting radicals such as HO in the presence of carbonate ions (141). In the latter, exemplified by the reaction of h on(II) with H2O2 and KHCO, the carbonate radical anion is probably a key intermediate and may account for many observations of weak chemiluminescence in oxidation reactions. 

Tri- and pentacoordinate phosphoms compounds often react by electron-pair mechanisms as demonstrated by the nucleophilic reactivity of the lone pair electrons in trivalent compounds, and the electrophilicity of the phosphoms atom in the pentavalent compounds. Some compounds also react by free-radical mechanisms. The theoretical and synthetic aspects of the chemistry of phosphoms compounds have been described (6—9). 

A free-radical reaction is a chemical process which involves molecules having unpaired electrons. The radical species could be a starting compound or a product, but the most common cases are reactions that involve radicals as intermediates. Most of the reactions discussed to this point have been heterolytic processes involving polar intermediates and/or transition states in which all electrons remained paired throughout the course of the reaction. In radical reactions, homolytic bond cleavages occur. The generalized reactions shown below illustrate the formation of alkyl, vinyl, and aryl free radicals by hypothetical homolytic processes. 

In a free radical, there is a third electron in the system. It cannot occupy the same orbital as the other two electrons and must instead be in an antibonding level. As a result, the transition state for migration is much less favorable than for the corresponding carboca-tion. 

When saturated steroidal ketones are reduced in ammonia, an alcohol is usually present to act as a proton donor and high yields of steroidal alcohols are obtained. Under these conditions, reduction probably proceeds by protonation of the radical-anion (or ketyl) (61), which results from a one electron addition to the carbonyl group, followed by addition of a second electron and proton. Barton has proposed that reduction proceeds via protonation of the dianion (62) arising from addition of two electrons to the carbonyl group. This proposal implies that the ketyl (61) undergoes addition of a second electron in preference to undergoing protonation by the... 

ESR can detect unpaired electrons. Therefore, the measurement has been often used for the studies of radicals. It is also useful to study metallic or semiconducting materials since unpaired electrons play an important role in electric conduction. The information from ESR measurements is the spin susceptibility, the spin relaxation time and other electronic states of a sample. It has been well known that the spin susceptibility of the conduction electrons in metallic or semimetallic samples does not depend on temperature (so called Pauli susceptibility), while that of the localised electrons is dependent on temperature as described by Curie law. 

The reaction of perfluoroalkyl iodides with electron donor nucleophiles such as sodium arene and alkane sulfinates in aprotic solvents results in radical addition to alkenes initiated by an electron-transfer process The additions can be carried out at room temperature, with high yields obtained for strained olefins [4 (equations 3-5)... 

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