Excitation allyl radicals

Deyerl and co-workers have also studied the photodissociation of the jet-cooled allyl radical produced in a flash pyrolysis source.148,149 The allyl radical is excited near 250 nm and H-atom product is detected by... 

Type 1 intrazeolite photooxygenation of alkenes has been also reported to give mainly allylic hydroperoxides (Scheme 42). In this process, the charge transfer band of the alkene—O2 complex within Na-Y was irradiated to form the alkene radical cation and superoxide ion. The radical ion pair in turn gives the allylic hydroperoxides via an allylic radical intermediate. On the other hand, for the Type II pathway, singlet molecular oxygen ( O2) is produced by energy transfer from the triplet excited state of a photosensitizer to 02. 

The discovery of 7r-allylic complexes is an exciting recent development in coordination chemistry. In these complexes the bonding of the allylic radical... 

The cation radical can undergo deprotonation to yield an allyl radical or nucleophilic attack by the solvent to produce a methoxyalkyl radical. Coupling of these radicals with the aromatic radical anion produces acyclic adducts. As an alternative, the anion radical can be protonated, ultimately giving reduction product. Thus, the degree of charge separation within the excited state complex dramatically influences the observable chemistry. 

Lee S-H, Mendenhall GD (1988) Relative yields of excited ketones from self-reactions of alkoxyl and alkylperoxyl radical pairs. J Am Chem Soc 110 4318-4323 Leitzke A, Reisz E, Flyunt R, von Sonntag C (2001) The reaction of ozone with cinnamic acids - formation and decay of 2-hydroperoxy-2-hydroxy-acetic acid. J Chem Soc Perkin Trans 2 793-797 Lodhi ZH, Walker RW (1991) Oxidation of allyl radicals kinetic parameters for the reactions of allyl radicals with H02 and 02 between 400 and 480 °C. J Chem Soc Faraday Trans 87 2361-2365 Martini M, Termini J (1997) Peroxy radical oxidation of thymidine. Chem Res Toxicol 10 234-241... 

The di-f-butylhydroxylamine and di-/-butylhydroxylamine ethers probably result from DTBN scavenging of radicals produced by hydrogen atom abstraction from the olefins by excited 3-ethoxyisoindolenone (50). The observed destruction of DTBN as a function of olefin structure is consistent with this mechanism. Based upon allyl radical stability and the statistical factor, excited 3-ethoxyisoindolenone should abstract hydrogen atoms more rapidly from tetraroethylethylene than from ds-2-butene. 

Benzene The patterns noted above for allyl radical are general for conjugated systems with an average of one electron per site. In all these cases, we will find that one of the combinations of the Kekule structures corresponds to a third and new structure (or to a set of different ones), and this arises from the fact that the Kekule structures of these systems share common determinant(s), the QC ones, that are doubled in one combination and eliminated in the other, as the two structures mix (recall Section 5.4 on aromaticity). As such, the two combinations will possess different bond-pairing features, which in the case of the excited-state combination will also indicate qualitatively the photochemical products expected from the molecule. 

The spin density distribution in the 2A2 excited state requires the derivation of all the contributing determinants as done for allyl radical. A full treatment is given in Exercise 8.5, while here we provide an approximate description. Already at the outset one can recall that the coefficient of the QC determinant in the excited state s wave function is zero, and we therefore expect very different spin density distribution than in the ground state. To proceed, we first express the resonance structures as products of the bonds and the odd electron. Thus... 

Write the Heisenberg Hamiltonian of allyl radical and diagonalize it. Then write the wave functions for the ground and first neutral excited state. Show that the excited state has a positive a spin density on the central atom, as discussed in Chapter 1. 

Different behavior is observed for the stable allyl radical, bis-(9-fluorenyl)-phenylmethyl radical (225) [139] (Scheme 19). Photolysis of this radical in poor H atom donating solvents is suggested to lead to the intermediate (226) which subsequently reacts with molecular oxygen yielding a variety of cleavage and oxygenation products. The excited radical lifetime in this case is less than 30 ps. 

The excited biradical produced in the primary step can readily isomerize into propene, which will contain sufficient excess energy to split off a hydrogen atom with the formation of the allyl radical ... 

The asterisk indicates some excited state of free radicals. The conversion of the free radical —CH2CHf- to —CH2CHCH3 could easily be supposed when one takes into consideration that the free radicals —CH2CHf might be produced as intermediate species in photo-induced radical conversion from an allylic radical to the free radical —CH2CHCH3 (71). 

Table 25.2 Equilibrium geometries for the ground and first excited state of the allyl radical Ci is the central carbon atom (bond distances in A)...

Table 25.3 Vertical excitation energies for the allyl radical (oscillator strengths within parenthesis) ...

The discussion of cis-trans photoisomerization of alkenes, styrene, stilbene, and dienes has served to introduce some important ideas about the interpretation of photochemical reactions. We see that thermal barriers are usually low, so that reactions are very fast. Because excited states are open-shell species, they present new kinds of structures, such as the twisted and pyramidalized CIs that are associated with both isomerization and rearrangement of alkenes. However, we will also see familiar structural units as we continue our discussion of photochemical reactions. Thus the triplet diradical involved in photosensitized isomerization of dienes is not an unanticipated species, given what we have learned about the stabilization of allylic radicals. 

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