Triplet ground state hydrogen atom abstraction

At this point let us briefly consider the relationship between the carbonyl triplet state and another system capable of hydrogen atom abstraction alkoxy radicals. A comparison of the differences and/or similarities between the reactivity of the carbonyl triplet and that of an alkoxy radical should indicate whether the triplet state behaves as a normal ground state radical or if electronic excitation imparts unique properties leading to reactions not characteristic of ground state radicals. 

Quinones undergo much interesting photochemistry. Triplet an-thraquinone lies 62.4 kcal above the ground state and readily abstracts hydrogen atoms from alcoholic solvents to yield semiquinone radicals.443... 

Reactions of the ground state, NH (X X ), play an important role in combustion processes. Triplet NH reacts with molecular hydrogen, water and CO2. Modem theoretical study demonstrates that reactions with H2 and H2O proceed via hydrogen atom abstraction. The NH abstracts hydrogen atoms from starting material, hydrazoic acid, and from hydrocarbons to form aminyl (NH2 ) and aUcyl radicals in spite of the fact that some reactions are endothermic, depending on the alkane. Absolute rate constants for many of these reactions have been measured in the gas phase. Triplet NH also reacts with aUcenes via formation of an intermediate triplet diradical which then decomposes into several reaction channels. ... 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

The parent nitrenium ion (NH2) is firmly established as a ground-state triplet both extensive ab initio calculations as well as PES experiments all agree that the singlet-triplet energy gap is 30 kcal/mol. There have been several investigations on its behavior in solution. Takeuchi et al. " showed that this species could be generated by photolysis of l-amino-(2,4,6-triphenylpyridinium) ion. These photolyses were carried out in the presence of various aromatic compounds. It was found that the triplet state abstracted hydrogen atoms from traps such as toluene... 

Galardy et al. (1973) introduced acetophenones and benzophenones for photoaffinity labeling (see also Martyr and Benisek, 1973 Katzenellenbo-gen et al., 1974). The properties of excited states generated from these and other a,P-unsaturated ketones are well understood (Turro, 1979). In most cases a triplet excited state is formed that abstracts a hydrogen atom from a donor yielding two radicals which subsequently couple. Unreacted excited species relax to the ground state and may be excited repeatedly until they react (Fig. 2.5). 

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