E.P.R. and ENDOR investigation of tempone nitroxide radical in a single crystal of tetramethyl-1,3-cyclobutanedione

The E.P.R. and ENDOR spectra of the free nitroxide radical tempone diluted in a single crystal of 2,2,4,4-tetramethylcyclobutanedione (TMCB) have been analysed and the g tensor, nitrogen hyperfine and quadrupole interaction tensors, and methyl protons tensors have been determined. It is found that the principal axes of the g tensor and of the tensors A N and Q N do not fully coincide. The negative sign of the quadrupole interaction tensor element Qzz where Z is along the direction of the nitrogen p orbital agrees with previous measurements on similar systems.