An ESR, ENDOR, and ELDOR study of tunneling rotation of a hindered methyl group in x-irradiated 2,2,5-trimethyl-1,3-dioxane-4,6-dione crystals

Electron spin resonance (ESR), electron nuclear double resonance (ENDOR), and electron-electron double resonance (ELDOR) spectra are reported for radical (I) in x-irradiated 2,2,5-trimethyl-1,3-dioxane-4,6-dione single crystals over the temperature range 10–64 K. The β-methyl group is observed to undergo tunneling rotation with a coalescence temperature of 30 K. The height of the hindering barrier has been deduced from the temperature dependence of the ESR spectrum and also from the solution to the Mathieu equation. All measurements are consistent with a tunneling splitting equal to 830±40 MHz.