Electron Spin Resonance Study on the Difference of Structure in Diastereomeric Nitroxyl Radicals

Electron spin resonance spectra of nitroxyl radicals bearing two asymmetric carbons were observed. The radicals were produced by the abstraction of a hydrogen atom from ethers and amines by photoexcited benzophenone, followed by spin trapping with phenyl-N-t-butylnitrone. The spectra showed the presence of two radical species assignable to diastereomers in most cases. The presence of isomers was seen more clearly in ESR spectra of radicals produced from ethers than those from amines. The origin of the different ESR parameters of the isomer was discussed in detail and it was concluded that the functional group on β-carbon acts as a group of proximity to nitroxyl function. The difference of the electronegativity of the atom in each functional group causes the difference of the extent of the separation of isomers. The effects of proximity groups on observed ESR parameters are concluded to be the same as the solvent effect.