ENDOR studies of the superhyperfine couplings of hydrogen-bonded protons. I. Carboxyl radical anions in irradiated L-alanine single crystals

The carboxyl radical anion trapped in an irradiated single crystal of L ‐alanine was studied. ENDOR techniques were used to clarify the origin of the transferred proton in this anion. The hyperfine tensor of the transferred proton and the superhyperfine tensors of the hydrogen‐bonded protons in the neighboring ${\mathrm{NH}}_3^{+}$ groups were determined. The results show that the transferred proton forms a normal covalent bond with the carboxyl oxygen atom and is located outside the COO plane. The direction of the O–H bond formed by the transferred proton is approximately parallel to one of the three hydrogen bonds in the undamaged crystal. This suggests that this proton is transferred across this particular hydrogen bridge. In addition, the large positive isotropic coupling of the transferred proton was accounted for by the structure of the protonated anion.