Triplet state of radical pair

Singlet and Triplet States of Radical Pairs and Their Mixing... 

CIDEP and CIDNP are due to nonequilibrium populations in the electron spin sub-levels of reacting radicals and in the nuclear spin sub-levels of reaction products, respectively. CIDEP and CIDNP have been successfully explained by the radical pair mechanism, according to which the singlet and triplet states of radical pairs can be mixed with each other through the Zeeman interaction... 

It is known that for phenyl alkyl ketones both the Type I and Type II reactions occur only from triplet state. The radical pair formed from a-cleavage undergo disproportionation resulting in Type I products or recombine to regenerate the ground state ketone. The latter process reduces the efficiency of Type I product formation. 

This review is divided into the following sections. First, EPR measurements of g-tensors of flavoproteins and the modulation of the principal values of g by the protein surroundings of the cofactor are discussed. Then, two recent examples of application of pulsed ENDOR spectroscopy will be reviewed, and, finally, time-resolved EPR spectroscopy, that is most favorably used to study photo-excited triplet states and radical pairs, will be introduced. 

The excited triplet states of quinones can be fairly readily populated by irradiation and nuclear polarization observed (Cocivera, 1968). Hydrogen atom abstraction leads to the relatively stable semiquinone radicals and, in alkaline media, radical anions. Recombination of radical pairs formed in this way can give rise to CIDNP signals, as found on irradiation of phenanthraquinone (20) in the presence of donors such as fluorene, xanthene and diphenylmethane (Maruyama et al., 1971a, c Shindo et al., 1971 see also Maruyama et al., 1972). The adducts are believed to have the 1,2-structure (21) with the methine proton appearing in absorption in the polarized spectrum, as expected for a triplet precursor. Consistently, thermal decomposition of 21 as shown in equation (61) leads to polarization of the reactant but now in emission (Maruyama... 

On the other hand, oxidation of a DNA base by a triplet state of the an-thraquinone (AQ5"3) generates a contact ion pair in an overall triplet state, and back electron transfer from this species to form ground states is prohibited by spin conservation rules. Consequently, the lifetime of the triplet radical ion pair is long enough to permit the bimolecular reaction of AQ- with 02 to form superoxide (02 ) and regenerate the anthraquinone. 

J.R. Bolton We have not carried out any experiments as yet on metalloporphyrins linked to quinones. The spin selection rules should be operative in the radical pair. The singlet state of the radical pair should be able to return to the ground state with no spin inhibition however, the triplet state of the radical pair can return to the ground state only via spin interconversion or via the triplet state of the porphyrin. 

The total quantum yield [4>cs(total)] for CS is decreased to 0.17 in dimethyl-formamide (DMF) due to the competition of the CSH from Fc-ZnP-H2F+-C6o (1.63 eV) to Fc-ZnP- -HzP-Cso (1.34 eV) versus the decay of Fc-ZnP-Fl2P -C6o to the triplet states of the freebase porphyrin (1.40 eV) and the Ceo (1.50 eV) [47]. In contrast to the case of most donor-acceptor-linked systems, the decay dynamics of the charge-separated radical pair (Fc -ZnP-H2P-C6o ) does not obey first-order kinetics, but, instead, obeys second-order kinetics [47]. This indicates that the mframolecular electron transfer in Fc -ZnP-H2P-C6o" is too slow to compete with the diffusion-limited inter-molecular electron transfer in solution. 

The primary donor triplet state 3P The triplet state of the primary donor is formed by recombination of the primary radical pair P,+4>X of prereduced bRCs in which the ET to the quinones is blocked ... 

The Primary Donor Triplet State iP7a0 If in the charge separation process electron-transfer in PS I beyond the first acceptor A0 is blocked by treatment with sodium dithionite at high pH and illumination, which reduces the iron-sulfur centres (F) and the quinone (A, the triplet state of the donor, 3P7ao, is obtained via radical-pair recombination from the triplet RP according to ... 

In photosynthesis radical-ions and triplet states of the pigments, radical-pairs and biradicals involving various chlorophylls and quinones, amino acid radicals, hemes in cytochromes, metal clusters of low and higher nuclearity and even coupled metallo-radical species have been observed. Thus the field of photosyn-... 

The triplet state of the geminate radical pair has three distinct levels (aa, /3/3, a/3 4- /3a) which are isoenergetic in the absence of any magnetic field. They interconvert very rapidly with the isoenergetic singlet state, so that the spin flip is fast and recombination is efficient. 

Figure 1.1 Examples of temperature dependences of rate constants for the reactions in which the low-temperature rate constant limit has been observed 1, hydrogen transfer in excited singlet state of molecule (6.14) 2, molecular reorientation in methane crystal 3, internal rotation of CH3 group in radical (7.42) 4, inversion of oxyranyl radical (8.18) 5, hydrogen transfer in the excited triplet state of molecule (6.20) 6, isomerization in the excited triplet state of molecule (6.22) 7, tautomerization in the ground state of 7-azoindole dimer (6.15) 8, polymerization of formaldehyde 9, limiting stage of chain (a) hydrobromi-nation, (b) chlorination, and (c) bromination of ethylene 10, isomerization of sterically hindered aryl radical (6.44) 11, abstraction of a hydrogen atom by methyl radical from a methanol matrix in reaction (6.41) 12, radical pair isomerization in dimethylglyoxime crystal (Figure 6.25).

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