Bunzo Eda

ESR Study of a Carboxyl Radical Trapped in a Single Crystal of Maleic Acid Irradiated at 77°K

The ESR evidence for the carboxyl radical, the existence of which is often assumed in organic solution chemistry as a reaction intermediate, has been obtained in a single crystal of maleic acid‐d2 (DOOC–CH=CH–COOD) irradiated at 77°K in the dark. The radical HH\/C=C/\O=CC–O·\//OD···O has been found to be formed by the removal of the deuteron attached to the side oxygen atom. The g tensor has been determined to be (2.0261, 2.0061, 2.0035). The unpaired electron of the radical occupies the in‐plane p orbitals of the carboxyl oxygen atoms, forming a σ electron radical. The carboxyl radical is believed to be produced from the positive primary by transferring the acidic proton to the neighboring molecule. It has been also found that the carboxyl radical fragments into the vinyl‐type radical HH\/C=C·/O=C\OD These results indicate that R·radicals often found in irradiated carboxylic acids (RCOOH) originate from the positive primary via the carboxyl radical (RCOO·).

ESR Spectra of γ‐Irradiated Polycrystalline Powders of Some Fluorine‐Containing Compounds

The powder spectra of γ‐irradiated salts and amides of some fluorinated acids were interpreted based on the spectral shapes of the α‐fluorine hyperfine‐coupling line. Because of the large anisotropy of α‐fluorine coupling tensor, it was possible to observe the shoulders corresponding to the maximum principal value of the coupling tensor. The separation between the wing patterns and the wing hyperfine structures made it possible to identify the radical species produced. The probable radicals were the ones formed by the removal of the atoms attached to the α carbon.

E.S.R. study of the electron trapping centre in a single crystal of maleic acid irradiated at 77 K

Evidence for an electron trapping centre produced by ionizing radiations in a single crystal of maleic acid has been obtained by electron spin resonance. When the crystal is irradiated at 77 K several paramagnetic species are produced. One of them is the species in which the unpaired electron occupies the delocalized π orbital and interacts with the two vinylene protons. The hyperfine tensors of the two vinylene protons, which are typical of α proton coupling, are (-13·0, -8·5, -3·2) and (-6·4, -5·2, -1·9)G, respectively. The g tensor is (2·0043, 2·0040, 2·0024). The principal directions of the hyperfine and g tensors give information about the electron trapping centre in which the framework of the molecule is preserved. The probable structure of this centre is the delocalized carboxyl anion although the possibility of its protonated form cannot be completely excluded.

Formation of allylic and hydrogen addition radicals in tiglic acid (αβ-dimethylacrylic acid) irradiated at 77 K as studied by ESR

Abstract Nearly equal amounts of allylic and hydrogen addition radicals are formed in tiglic acid [ CH 3 CH = C ( CH 3 ) COOH ] irradiated at 77 K by loss of a hydrogen atom from the β methyl group and by addition of a hydrogen atom to the β carbon atom, respectively. Unlike usual unsaturated carboxylic acids, the origin of the added hydrogen atom is likely from the C-H hydrogen rather than the acidic one, because deuterium substitution of the acidic hydrogen does not result in deuterium addition. In β-methyl substituted acrylic acids, deprotonation of the primary cation seems to take place at the β methyl group forming allylic radicals and the detached proton adds to the primary anions forming hydrogen addition radicals [ CH 3 CH 2 - C ( CH 3 ) COOH ], although a possibility of homolytic scission of the C-H bond in the β methyl group followed by the hydrogen atom addition to form the above mentioned two radicals cannot be excluded. The drastic difference from the acidic proton addition to the molecular anion in many unsaturated carboxylic acids may be ascribable to the existence of β methyl group which leads to the formation of persistent allylic radicals.

ESR Study of the Vinyl‐Type Radical Trapped in a Single Crystal of Maleic Acid Irradiated at 77°K

The substituted vinyl radical, DOOC–CH=ĊH, trapped in an irradiated single crystal of maleic acid‐d2 has been studied by ESR to elucidate the geometrical and electronic structure of the vinyl‐type radical. The hyperfine tensors for the 13C and β proton couplings have been determined to be (+133.8, +75.3, +62.2)G and (+59.5, +57.5, +56.8)G, respectively, in addition to the α proton coupling and g tensors preliminarily reported. All the magnetic tensors determined are consistent with the planar structure of the vinyl‐type radical in which the unpaired electron occupies the in‐plane sp hybrid orbital. The 13C hyperfine tensor indicates the relatively large spin density in the C2pσ orbital which is consistent with the small s character of the vinyl‐type radical. The relative orientation of the hyperfine tensor of the α proton to that of 13C gives the clear evidence for the bent structure of the radical carbon atom. The dipolar tensor of the α proton coupling has been calculated using the spin distribution and...