ENDOR studies of the superhyperfine couplings of hydrogen-bonded protons. IV. Carboxyl radical anions in irradiated glycine and α-amino isobutyric acid

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Abstract
The carboxyl radical anions trapped in irradiated single crystals of α‐glycine and α‐amino isobutyric acid have been studied by ENDOR. To elucidate the controlling factors for the selective proton transfer from the neighboring NH3 groups to the anion across the hydrogen bridge, the superhyperfine coupling tensors of the protons in the neighboring NH3 groups have been determined. The results suggest that two of the three amino protons hydrogen bonded to the COO group of the anion are located in the middle of the hydrogen bridge or jumping between the nitrogen and oxygen atoms. The latter interpretation seems to be more probable. The remaining one is located in the neighboring NH3 group. The following controlling factors suggested in our previous paper were confirmed by comparing the results with the reported crystallographic data: firstly, the protons participating in the hydrogen bond, which is formed in the direction of the unpaired electron orbital of the anion, are easily transferred to the anion side; and secondly, the hydrogen bond distance which is shorter than the others is favorable for the transfer.