Electron Spin Resonance of an Irradiated Single Crystal of Alanine: Second-Order Effects in Free Radical Resonances

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Abstract
The electron spin resonance of single crystals of l‐ and d‐alanine has been observed at T=300°K and analyzed for different orientations of the crystal in the magnetic field and at several microwave frequencies ranging from 9 kMc/sec to 34 kMc/sec. The stable free radical produced by the irradiation is proved to be of the form CH3CHR, where R is a group which has no nuclei with detectable coupling. The hydrogens of the CH3 group of the radical are shown to have equivalent, isotropic coupling of 26 gauss each, essentially independent of the frequency of observation. This CH3 group coupling is interpreted as arising from s orbital spin density of the hydrogens, via hyperconjugation. The hydrogen of the CH group has both an isotropic, Fermi term, Af=20 gauss, arising from s orbital density on the hydrogen, and an anisotropic term Aμ=7 gauss arising from dipole‐dipole interaction of the proton moment with the electron spin density, ρC, on the carbon. Although the signs of Af and Aμ could not be learned, they are shown to be of opposite sign. Hence the spin density is negative on either H or C. Principal values of the CH coupling are A1=7 gauss, A2=A3=27 gauss. The value of ρC is shown to be 0.80 approximately. For certain orientations of the crystal the CH coupling becomes equal to the CH3 coupling, and a quintet pattern is observed. Interesting second‐order transitions are observed which for the [001] orientation become in the region of 24 kMc/sec as strong as the normal first‐order transitions. A general theory is developed which accounts satisfactorily for these second‐order effects which are probably of consequence in the electron spin resonance patterns of numerous other free radicals trapped within solids.