Singlet radical pairs, potential

Methylquinolinium ion derivatives are reduced regioselectively to 1,4-dihydroqui-nones by (TMS SiH under photochemical conditions42. Mechanistic studies demonstrated that the reactions are initiated by photoinduced electron transfer from silane to the singlet excited states of 1-methylquinolinium ion derivatives to give the silane radical cation—quinolinyl radical pairs, followed by hydrogen transfer in the cage to yield 1,4-dihydroquinones and silicenium ion. The one-electron oxidation potential of (TMS SiH is 1.30 V42. 

For non-electrophilic strong oxidants, the reaction with an alkane typically follows an outer-sphere ET mechanism. Photoexcited aromatic compounds are among the most powerful outer-sphere oxidants (e.g., the oxidation potential of the excited singlet state of 1,2,4,5-tetracyanobenzene (TCB) is 3.44 V relative to the SCE) [14, 15]. Photoexcited TCB (TCB ) can generate radical cations even from straight-chain alkanes through an SET oxidation. The reaction involves formation of ion-radical pairs between the alkane radical cation and the reduced oxidant (Eq. 5). Proton loss from the radical cation to the solvent (Eq. 6) is followed by aromatic substitution (Eq. 7) to form alkylaromatic compounds. 

Examine the highest-occupied molecular orbital (HOMO) of singlet methylene. Where is the pair of electrons, inplane or perpendicular to the plane Next, examine the electrostatic potential map. Where is the molecule most electron rich, in the o or the 7t system Where is the most electron poor Next, display the corresponding map for triplet methylene. Which molecule would you expect to be the better nucleophile The better electrophile Explain. Experimentally, one state of methylene shows both electrophilic and nucleophilic chemistry, while the other state exhibits chemistry typical of radicals. Which state does which Elaborate. 

The ground state (0 kJ/mol) for the CL molecule is represented by the term symbol 3v . The first excited state (92 kJ/mol above the ground state) is a 1 singlet (electrons spin paired with both electrons in either the n x or the n y level). The 1 v state with paired spin electrons, one each in the 7i v and n y levels, is the next excited level 155 kJ/mol above the ground state. Reduction of 02 by one electron yields the superoxide ion (02), a radical anion. Reduction by two electrons yields the peroxide ion, (02 ). Bond lengths and bond orders for these are given in Table 4.2. As noted in equation 4.2, the reduction potential for 02 in the presence of protons is thermodynamically favorable. Therefore, reversible binding of O2 to a metal can only be achieved if competition with protons and further reduction to superoxide and peroxide are both controlled.8... 

Although Ceo is easily reduced, it is very difficult to oxidize [46, 53, 54, 72], The only definitive electrochemical oxidation of Cgo occurs at a potential of -F 1.76 V vs SCE in benzonitrile, and is irreversible [54]. The radical cation was reported to be produced by y-irradiation at 77 K in a glass, and to absorb near 980 nm [65, 66]. Attempts to generate the radical cation (Ceo) by electron transfer to singlet excited dicyanoanthracene, which has a reduction potential near + 2.0 V [73] were unsuccessful. This method has been used to generate, for example, trans-stilbene radical cation [73, 74]. The ion pair probably does not... 

The excited singlet state of iV-methylacridinium hexafluorophosphate (MA ) has a reduction potential of 2.31 V vs SCE, [75, 76] sufficient to oxidize Ceo) and also has a longer singlet lifetime than DCA. Reduction of the cationic sensitizer produces a neutral radical and a radical cation rather than an ion pair so that there should be no coulombic attraction to hinder the dissociation of the electron-transfer pair [76]. 

---