Investigations of structure and conformation. Part 9. Electron spin resonance evidence for conformational preferences and hindered rotation about ·C–CH3 in some αα-dioxygen-substituted alkyl radicals. Determination of the barriers to rotation

Abstract

The e.s.r. spectra of some α-dioxygen-substituted α-methylalkyl radicals in fluid solution show a marked broadening of the inner (m_I±½) lines of the methyl proton hyperfine splitting pattern as the temperature is lowered. Results for the appropriate dioxolanyl and dioxanyl species trapped in solid matrices show that the preferred conformation of the methyl group is one in which one β-proton splitting is large (ca. 3.0 mT). INDO calculations support an interpretation in which this β-proton eclipses the orbital of the unpaired electron in a type of radical with considerable bending at the tervalent carbon atom. The barrier to rotation about ·C–CH₃ in these radicals is estimated as ca. 8 kJ mol^–1. α-Aryl αα-dioxygen-substituted radicals [e.g.·C(C₆H₅)(OMe)₂] are found to be planar at the radical centre.