130GHz ESEEM induced by electron–electron interaction in biradical

A three-pulse stimulated ESE at 130 GHz (D-band) of nitroxide biradical in a molecular glass shows envelope modulation (ESEEM) when the time separation between the first and the second pulses is varied. This ESEEM originates from relaxation-induced flips of the spin partner during the mixing period between the second and third pulses. These flips alternates the local dipolar field in which the resonant spins precess before second and after third pulses. The ESEEM is different for different spectral positions, due to orientation selectivity. For toluene glassy solution at 35 K Fourier transform shows pronounced peak reflecting singularity of the Pake resonance pattern. Increasing temperature up to 80 K results in appearance of a strong additional peak ascribed to alteration of the resonance field induced by methyl group reorientation.