Hydrogen atom - methyl radical pairs

The U-hf splittings at the Attis = 1 transition were further split by the fine structure (fs) with axial symmetry due to the electron dipole-dipole coupling. Three different sets of hefs splitting, d , were resolved in the spectra of the H—CH3 radical pairs and were evaluated by the ESR spectral simulation method to be d = 4.2, 4.9, and 5.1 mT for pairs I, II and III, respectively [76]. Applying the point dipole approximation, the d value can be related to the separation distance, R, between two radicals by the following equation [37b, 89]  

The present ESR studies using the selectively D-labeled methanes, CD4 and CH2D2, unambiguously revealed that the hydrogen atom and the methyl radical present in pairs originate from the same methane molecule, suggesting that the radical pair formation is caused by the homolytic C— H bond scission via the electronically excited methane molecule (CHp. The H-atoms initially formed may 

We close this section by noting that Knight et al. have reported ESR spectra of more simple triplet radical pairs of and D - D trapped in rare gas 

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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].

R.L. Sweany, University of New Orleans I was surprised at seeing your report of a 2D Cu atom being able to abstract a hydrogen atom from methane, but, of course, the copper atom is "hot". I wonder if you see methyl take back its hydrogen atom after photolysis or does the radical pair collapse to give HCuCH3 ... 

We have seen in the last chapter that electrons have a spin angular momentum corresponding to the quantum numbers -f i and In most molecules the electron spins are paired, but some molecules contain an odd number of electrons there is therefore at least one unpaired electron spin. An example of an ordinary molecule with an unpaired electron is nitric oxide (NO), which has an odd number (15) of electrons. Of particular interest are the free radicals, such as the methyl radical (CH3). This radical is produced if a hydrogen atom is abstracted from a methane (CH4) molecule, and its Lewis structure is... 

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