Radicals hybridization

Sato S, Tsunoda M, Suzuki M, Kutsuna M, Takido-uchi K, Shindo M, Mizuguchi H, Obara H, Ohya H (2009) Synthesis and spectral properties of polymethine-cyanine dye-nitroxide radical hybrid compounds for use as fluorescence probes to monitor reducing species and radicals. Spectrochim Acta A 71 2030-2039... 

Oxman et al., smdied controlled, sequentially curable cationic/free radical hybrid photopolymerization of diepoxide/acrylate hybrid material with the aid of photodifferential scanning calorimetry. The polymerizations were carried out in the presence of various concentrations of either ethyl-4-dimethylamino benzoate or 4-tert-butyl-N,N,-dimethylaniline as electron donors and camphoquinone/diphenyliodonium hexafluoroantimonate as the sensitizing system. The results showed that the free-radical acrylate reactions always precede the cationic epoxy polymerizations. 

The representation of non-bonding orbitals on an atom again uses the concept of. T-systems, though they may have any kind of hybridization (p, sp etc.), In Figure 2-56 the three possibilities arc shown lone pairs, radicals, and orbitals without electrons can be accommodated by this eoneept. 

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... 

An alkyl radical is neutral and has one more electron than the corresponding carbocation Thus bonding m methyl radical may be approximated by simply adding an electron to the vacant 2p orbital of sp hybridized carbon m methyl cation (Figure 4 19a) Alternatively we could assume that carbon is sp hybridized and place the unpaired elec tron m an sp orbital (Figure 4 9b)... 

Of the two extremes experimental studies indicate that the planar sp model describes the bonding m alkyl radicals better than the pyramidal sp model Methyl rad ical IS planar and more highly substituted radicals such as tert butyl radical are flattened pyramids closer m shape to that expected for sp hybridized carbon than for sp ... 

Using an sp hybridized carbon for the carbon that has the unpaired electron make a molecular model of the free radical intermediate in this reaction... 

Vinyl ethers can also be formulated with acryHc and unsaturated polyesters containing maleate or fumarate functionaHty. Because of their abiHty to form alternating copolymers by a free-radical polymeri2ation mechanism, such formulations can be cured using free-radical photoinitiators. With acryHc monomers and oligomers, a hybrid approach has been taken using both simultaneous cationic and free-radical initiation. A summary of these approaches can be found in Table 9. 

Like the 5/) -hybridized carbons of carbocations and free radicals, the sp -hybridized carbons of double bonds are electron attracting, and alkenes are stabilized by substituents that release electrons to these carbons. As we saw in the preceding section, alkyl groups are better electron-releasing substituents than hydrogen and aie, therefore, better able to stabilize an alkene. 

The vinyl H2C=CH radical can be produced by cleavage of a C-H bond in ethene, and has been studied in the gas phase. The unpaired electron clearly occupies a carbon sp hybrid orbital, to lapse into the language of descriptive organic chemistry, but there are regions of space where the, 6-spin electrons have... 

Meanwhile, it was found by Asai and colleagues [48] that tetraphenylphosphonium salts having such anions as Cl, Br , and Bp4 work as photoinitiators for radical polymerization. Based on the initiation effects of changing counteranions, they proposed that a one-electron transfer mechanism is reasonable in these initiation reactions. However, in the case of tetraphenylphosphonium tetrafluoroborate, it cannot be ruled out that direct homolysis of the p-phenyl bond gives the phenyl radical as the initiating species since BF4 is not an easily pho-tooxidizable anion [49]. Therefore, it was assumed that a similar photoexcitable moiety exists in both tetraphenyl phosphonium salts and triphenylphosphonium ylide, which can be written as the following resonance hybrid [17] (Scheme 21) ... 

To see why allylic radicals are so stable, look at the orbital picture in Figure 10.3. The radical carbon atom with an unpaired electron can adopt sp2 hybridization, placing the unpaired electron in a p orbital and giving a structure that is electronically symmetrical. The p orbital on the central carbon can therefore overlap equally well with a p orbital on either of the two neighboring carbons. 

Radicals with very polar substituents e.g. trifluoromethyl radical 2), and radicals that arc part of strained ring systems (e.g. cydopropyl radical 3) arc ct-radicals. They have a pyramidal structure and are depicted with the free spin resident in an spJ hybrid orbital. nr-Radicals with appropriate substitution are potentially chiral, however, barriers to inversion are typically low with respect to the activation energy for reaction. 

Radicals with adjacent Jt-bonds [e.g. allyl radicals (7), cyclohexadienyl radicals (8), acyl radicals (9) and cyanoalkyl radicals (10)] have a delocalized structure. They may be depicted as a hybrid of several resonance forms. In a chemical reaction they may, in principle, react through any of the sites on which the spin can be located. The preferred site of reaction is dictated by spin density, steric, polar and perhaps other factors. Maximum orbital overlap requires that the atoms contained in the delocalized system are coplanar. 

Radical additions are typically highly exothermic and activation energies are small for carbon30-31 and oxygen centered32,33 radicals of the types most often encountered in radical polymerization, Thus, according to the Hammond postulate, these reactions are expected to have early reactant-like transition states in which there is little localization of the free spin on C(J. However, for steric factors to be important at all, there must be significant bond deformation and movement towards. sp hybridization at Cn. 

The photoelectron spectrum of methane shows two bands, at 23 and 14 eV, and not the single band we would expect from the equivalency of the four C—H bonds. The reason is that ordinary sp hybridization is not adequate to explain phenomena involving ionized molecules (such as the CH4" radical ion, which is left behind when an electron is ejected from methane). For these phenomena it is... 

Addition to conjugated systems can also be accomplished by any of the other three mechanisms. In each case, there is competition between 1,2 and 1,4 addition. In the case of nucleophilic or free-radical attack, the intermediates are resonance hybrids and behave like the intermediate from electrophilic attack. Dienes can give 1,4 addition by a cyclic mechanism in this way ... 

Fig. 4a, b General models for (a) Jt-type and (b) o-type diradicals, in which the radical orbitals are mainly of the p-character and sp hybrid, respectively... 

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