Anisotropic hyperfine interaction in the electron spin resonance spectrum of the methyl radical trapped in CH3COONa⋅3D2O crystal at low temperatures

Abstract

The electron spin resonance spectrum of the methyl radical produced by γ irradiation of CH3COONa⋅3D2O single crystal at 77 K has been measured at low temperatures. The spectrum observed at below 24 K consisted of eight lines with approximately equal intensities, instead of the four lines (1:3:3:1) at higher temperatures, due to the quantum tunneling of the rotational states. The transitions at low temperatures were angular dependent upon the applied field. From the analyses of the angular dependence of the spectrum with the aid of the theoretical calculation of the transition field for the tunneling methyl radical, the following anisotropic hyperfine tensor for the proton was determined (MHz): Axx=−31.4, Ayy=−93.7, Azz=−60.3, a (iso) =−61.8. These parameters provide the first experimental assessment of the well-known theoretical results on the proton hyperfine anisotropy in the free methyl radical.