Radicals intersystem crossing

In the absence of intersystem crossing (ISC) the (n, r ) state can react only to give the linear acyl radical. Intersystem crossing followed by conversion into the bent acyl radical is favored, however, since it is associated with a transition of an electron between two mutually orthogonal p orbitals. (Cf. Example 1.8.) The barrier for a cleavage from the state is con-... 

By examining the expression for Q ( equation (B1.16.4)). it should now be clear that the nuclear spin state influences the difference in precessional frequencies and, ultimately, the likelihood of intersystem crossing, tlnough the hyperfme tenn. It is this influence of nuclear spin states on electronic intersystem crossing which will eventually lead to non-equilibrium distributions of nuclear spin states, i.e. spin polarization, in the products of radical reactions, as we shall see below. 

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 second type of photoinitiators, ie, those that undergo electron transfer followed by proton transfer to give free-radical species, proceed as follows, where is the rate constant for intersystem crossing. 

Section V.D described the competition of two pathways in the H2 + CO molecular channel. There are also multiple pathways to the radical channel producing H + HCO. In aU cases, highly vibrationally excited CH2O is prepared by laser excitation via the So transition. In the case of the radical channel discussed in this section, multiple pathways arise because of a competition between internal conversion (S o) and intersystem crossing ( i T ), followed by evolution on these electronic states to the ground-state H + HCO product channel. Both electronic states So and Ti correlate adiabatically with H + HCO products, as shown in Fig. 7. 

Anthraquinones are nearly perfect sensitizers for the one-electron oxidation of DNA. They absorb light in the near-UV spectral region (350 nm) where DNA is essentially transparent. This permits excitation of the quinone without the simultaneous absorption of light by DNA, which would confuse chemical and mechanistic analyses. Absorption of a photon by an anthraquinone molecule initially generates a singlet excited state however, intersystem crossing is rapid and a triplet state of the anthraquinone is normally formed within a few picoseconds of excitation, see Fig. 1 [11]. Application of the Weller equation indicates that both the singlet and the triplet excited states of anthraquinones are capable of the exothermic one-electron oxidation of any of the four DNA bases to form the anthraquinone radical anion (AQ ) and a base radical cation (B+ ). 

Fig. 1 Schematic mechanism for the long-distance oxidation of DNA. Irradiation of the anthraquinone (AQ) and intersystem crossing (ISC) forms the triplet excited state (AQ 3), which is the species that accepts an electron from a DNA base (B) and leads to products. Electron transfer to the singlet excited state of the anthraquinone (AQ 1) leads only to back electron transfer. The anthraquinone radical anion (AQ ) formed in the electron transfer reaction is consumed by reaction with oxygen, which is reduced to superoxide. This process leaves a base radical cation (B+-, a hole ) in the DNA with no partner for annihilation, which provides time for it to hop through the DNA until it is trapped by water (usually at a GG step) to form a product, 7,8-dihydro-8-oxoguanine (8-OxoG)...

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>. 

Nonthermal Microwave Effects - Intersystem Crossing in Radical-recombination Reactions... 

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