ENDOR studies of the superhyperfine couplings of hydrogen-bonded protons. III. Protonated radical anions in irradiated L-valine hydrochloride monohydrate

The so-called carboxyl radical anion trapped in irradiated single crystals of L-valine hydrochloride monohydrate has been studied by ENDOR. The spectra exhibited two OHβ proton couplings, indicating that the anion is protonated to form a radical of the type R–Ċ(OH)2. The coupling tensors and their orientations clearly indicate that the amino proton in the neighboring NH3 group is transferred to the anion through the intermolecular hydrogen bond. Combining the results with those previously obtained for L-alanine and succinic acid, the variation of the dipolar tensor elements with the isotropic values has been examined for the OHβ proton couplings in this type of radical. It is then shown that the anisotropic coupling is approximately independent of the dihedral angle of the O–Hβ bond. The dipolar tensors were also calculated as a function of the dihedral angle, and the results are in good agreement with the observations.