Tumbling of an adsorbed nitroxide using rapid adiabatic passage

In order to extend electron spin resonance (ESR) studies of the tumbling of adsorbed nitroxide free radicals into the regime of slower motions we have employed rapid adiabatic passage (RAP) dispersion methods. We report combined observations of the temperature variations of ESR absorption and RAP dispersion signals for di-tert-butyl nitroxide (DTBN) dissolved in sec-butylbenzene and for DTBN adsorbed on silica. Motion of the adsorbate is observed over a large temperature range, and the combination of ESR techniques shows that surface nitroxides exhibit a broad distribution correlation times and a range of activation energies for motion.