Triplet radical pair

Postulate (ii) derives from the proposition that in any elementary process a singlet product will be more readily formed from a singlet reactant than from a triplet reactant, since multiplicity changes are of low probability. A triplet reactant could of course be converted to a triplet product but in most cases this is unlikely from purely energetic considerations. Consequently the components of triplet radical pairs tend to separate. 

Observation of spin-polarized products resulting from these radical pairs by the method of chemically induced dynamic nuclear polarization (CIDNP)<67) was accomplished by photolysis in the probe of an NMR spectrometer using perfluoromethylcyclohexane as solvent. The results obtained were consistent with nuclear spin polarization steps involving radical pairs formed from dissociated radicals and also directly from excited states, although the former could not be detected in carbon tetrachloride, probably due to radical scavenging by the solvent. It was not possible to determine the fraction of the reaction proceeding by singlet and triplet radical pairs.<68)... 

The a,( -unsaturated aldehyde 452 is generated from the unstable spiro-oxetane 451, and hydrogen abstraction from the aldehydic C-H bond by 3449 gave a triplet radical pair 453 and 454. Intersystem crossing and radical recombination followed by intramolecular nucleophilic attack of the hydroxyl group toward the ketene functionality furnish the diastereomeric products 54 and 55 (Scheme 102) <20000L2583>. 

The fates of the radical ion pairs produced upon electron transfer depends on the nature of their production. As already mentioned, the Bp DMA" com formed from irradiation of the ground-state CT complex. Bp - DMA, is suggested by Mataga and co-workers [24] to decay only by febet, on a timescale of 85 ps. Diffusional separation to solvent separated radical ion pairs or proton transfer within Bp -DMA com are not kinetically competitive. The triplet CRIP Bp -I- DMA" ip has two decay pathways that occur on the picosecond timescale. The first process is proton transfer, fept, to generate a triplet radical pair, BpH-l- DMA ] (Scheme 2.3). In acetonitrile, this occurs with a rate constant of fept of 1.3 x 10 s [43]. The second process leading to the decay of the CRIP is diffusional separation to the SSRIP, kips, which occurs with a rate constant of 5 x 10 s (Scheme 2.3) [43]. Thus the efficiency of the... 

Observation of emission and absorption in the NMR benzylic proton signal from an irradiated solution of diphenyldiazomethane in toluene was the first example showing the importance of the CIDNP technique. Triplet diphenylcarbene generated by photolysis of diphenyldiazomethane in toluene abstracts a hydrogen atom to generate the triplet radical pair, which either recombines to give 31, or diffuses apart, ultimately to produce dimers of each fragment (32, 33, Scheme 9.7,... 

The CIDNP experiments have been carried out to demonstrate an intervention of triplet radical pairs in many hydrogen abstraction reactions involving triplet car-benes. The following examples in reactions with halo compounds and ethers are of particular interest. 

The manifestations of singlet interactions need not be subtle for example, attempts to prepare 1,3-cyclohexadiene photodimers using naphthalene as a sensitizer will give an infinitesimal yield.156 Moreover, the reported159 use of triphenylene to form triplet radical pairs from azo compounds is incorrect since only singlet radical pairs are formed.160... 

We consider first how surfaces which are themselves not photosensitive can perturb chemical reactivity. First, the surface can influence diffusional motion of adsorbed substrates, intermediates or products. With preadsorbed substrates, one can probe the nature of motion of intermediates generated on the surface and search for differences in reactivity caused by surface confinement . When several photochemical precursors to benzyl radicals, e.g., benzyl phenylacetate, a dibenzyl ketone, or a dibenzyl sulfone, are irradiated as adsorbates on dry silica gel, singlet and triplet radical pairs are generated, Eq. (7). The extent of radical recombination observed requires... 

By measuring zero-field splitting (zfs) from electron-electron dipolar coupling in triplet radical pairs and proton hyperfine splitting (hfs), Segmuller developed a detailed picture of the motions of these radicals. The sequence of... 

Activation of elemental metals by mechanical methods (Sec. 3.4) in the presence of organic acceptor molecules in solid phase, described in detail [14], allows us also to obtain metal-polyradical complexes [206,216-219]. The formed radical-pair species have some unusual properties [217,218]. Compared with triplet radical pairs generated photochemically with the same donor-acceptor composition, these mechanically induced species appear to be much more stable [216]. As an example... 

The high value for the quenching of 3,4-dimethoxyacetophenone by phenol suggests that it is probable that within the lignin structure hydroxyl groups are able to quench carbonyls by a static mechanism to yield phenoxy-ketyl radical pairs which decay on a timescales faster than the time resolution of our laser flash photolysis apparatus. Intersystem crossing rate constants for triplet radical pairs in the restricted environments of micelles have been demonstrated to be of the order of 2 -5 x 106 s-1 (25, 24). However, in the lignin matrix where diffusional processes are likely to be... 

Note that in all the cases considered in Table 2 the ethyl protons of the initial Et3GeCOPh demonstrate positive polarization (A). Therefore, the analysis of these effects in accordance with the existing rules11 allows us to conclude that partially reversible photodecomposition of the ketone both in the presence and in the absence of the radical traps occurs from the triplet excited state with the formation of the triplet radical pair comprised of Et3Ge and COPh radicals. The analysis employed the following -factor and hyperfine interaction values of the radicals g(Et3Ge ) = 2.0089, g( COPh) = 2.0008 and Ah(CH2) < 0.5 mT (for Et3Ge )12. 

---