Dynamics and spin relaxation of tempone in a host crystal. An ENDOR, high field EPR and electron spin echo study

We studied in the temperature range 4-300 K the dynamics and the electron spin echo decay of the radical 4-oxo-2,2,6,6,-tetramethyl-1-piperidinyloxyl (tempone) doping substitutionally a single crystal of the diketone 2,2,4,4-tetramethyl-cyclobutan-1,3-dione. Electron nuclear double resonance (ENDOR) and high field electron paramagnetic resonance (EPR) spectra (110 GHz) displayed three types of motion: interconversion between twisted-crossover conformations, rotation of the methyl groups and libration of the radical in the crystal matrix. At room temperature all the motions are fast, and they reach the slow motion regime each at a different temperature, being all slow below 80 K. An approximate value of 9 kJ mol(-1) for the energy barrier hindering the rotation of the tempone methyl groups is estimated. At low temperature their motion is too slow to give any contribution to the echo decay. The echo decays as a linear exponential in the range 100-300 K, due to the radical motions. At T<100 K the interaction with the methyl protons of the matrix is the dominant dephasing mechanism, and the echo decays as a gaussian. At T < 20 K it depends on the square root of the time. Echo decays and electron spin echo envelope modulations (ESEEM) are simulated by using the available theoretical models. A damping of ESEEM superimposed to the echo decay is discussed.