Double-Resonance Studies of the Conformations of the Free Radical in Irradiated Alpha-Aminoisobutyric Acid

The technique of fast-passage double resonance (ENDOR) is applied to the study of the three conformations of the free radical found when warming single crystals of α-aminoisobutyric acid, x irradiated at 77°K. The ENDOR measurements, made at 4.2°K, allow the absorption signals for the protons of the two methyl groups to be distinguished at all crystal orientations. Both methyl groups in the first two conformations and one group in the third conformation are found to have a single representative ENDOR line, indicating the group's rotation. The hyperfine tensors for these groups are found to depart from the generally assumed axial symmetry. The nonrotating methyl group in the last conformation permits an accurate determination of the parameters in the coupling equation A=B0+B cos2θ. The experimental values are B0=3.2 Mc/sec and B=123.3 Mc/sec.