Radical pair, intersystem

The spin properties of charge-separated ion pairs can also be exploited for the purposes of all optical switching. Radical pair intersystem crossing (RP-ISC) of the form [D+ -A- ] D+ -A- ] to yield the spin-correlated triplet state is observed in... 

In the emulsion-polymerization of methyl methacrylate by carbonyl initiators, intramicellar processes have been found to reduce the conversion rate. On the other hand, the application of weak magnetic fields has been found to increase the conversion of emulsion-polymerized vinyl monomers using triplet ketone sensitizers. Here external magnetic fields will decrease the efficiency of triplet-to-singlet radical-pair intersystem crossing within the micelles and thus increase the function of radical pairs that escape without terminating the polymer chains. 

Since Ag is positive and is negative, Q is larger for the p state than for the a state. Radical pairs in the p nuclear spin state will experience a faster intersystem crossing rate than those in the a state with the result that more RPs in the p nuclear spin state will become triplets. The end result is that the scavenging product, which is fonned primarily from triplet RPs, will have an excess of spins in the p state while the recombination product, which is fonned from singlet RPs, will have an excess of a nuclear spin states. 

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 theory underlying this effect depends critically on two selection principles (1) the nuclear spin dependence of intersystem crossing in a radical pair, and (2)... 

The theory of CIDNP depends on the nuclear spin dependence of intersystem crossing in a radical (ion) pair, and the electron spin dependence of radical pair reaction rates. These principles cause a sorting of nuclear spin states into different products, resulting in characteristic nonequilibrium populations in the nuclear spin levels of geminate (in cage) reaction products, and complementary populations in free radical (escape) products. The effects are optimal for radical parrs with nanosecond lifetimes. 

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... 

The reaction starts with excitation of the quinone, followed by intersystem crossing and electron transfer from the thiophene to the triplet excited quinone. The ion radical pair collapses to a biradical which loses a chlorine and a hydrogen atom. Yields are high (65-78%) when R1 = halogen and R2 = H, fair (57%) when R1 = R2 = H and poor (2-17%) when R1 = H and R2 = halogen. The regioselectivity has been explained on the basis of calculated electron densities in the cation radicals of thiophenes. 

Homolytic bond cleavage as a result of a - a or a - ct photoexcitation produces a singlet cation-radical/radical pair that can recombine to produce starting material, molecular rearrangement, undergo intersystem crossing to the triplet pair or ET to form the benzyl carbocation. A possible description of the a/cr excited state is represented below. The rate of bond cleavage must be competitive with back ET. 

The photolyses of dlbenzyl ketones in aqueous micellar solution have been shown to greatly enhance both geminate radical pair recombination and the enrichment of in recovered ketone compared to homogeneous solution. These observations have been attributed to the combined effects of the reduced dimensionality imposed by mlcelllzatlon and hyperflne induced intersystem crossing In the geminate radical pairs. This latter effect is the basis of Chemically Induced Dynamic Nuclear Polarization (CIDNP), a phenomenon which is well known in homogeneous solution. 

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