Dynamic nuclear polarization by thermal mixing of cyclohexane in its plastic phase

Evidence concerning the formation of a heterogeneous solid solution in the plastic phase of cyclohexane doped with a nitroxide free radical from magnetic resonance studies

“…The aim of the present paper is to reconsider the DNP and SLR data of references [9][10][11] in the light of the new results of references [13][14][15]. This new analysis will essentially lead to the conclusions labelled (i) and (ii).…”

“…In previous papers, magnetic resonance studies of the plastic phase of cyclohexane doped with the nitroxide free radicals TANO and TANANE have been reported [9][10][11][12][13][14][15]. In this system, in addition to E.P.R.…”

“…It is currently used in biology [6] and in the field of liquid crystals [7,8], but it has been seldom used for the study of plastic crystals. In plastic cyclohexane [9][10][11][12][13][14][15], camphene [16], camphor [17], using suitable free radicals at low concentrations, the E.P.R. lines are found to be narrow, suggesting (i) that the radicals are diluted in the medium and (ii) that the motion is sufficiently rapid, two features which show that spin labelling may be an interesting method for studying these systems.…”

“…spectra and some aspects of the DNP results are consistent with the idea of isolated free radicals replacing cyclohexane molecules in the crystalline lattice, and rotating very rapidly, some other aspects, more apparent in the high temperature range, do not support such a simple and somewhat naive picture. In particular, the existence of DNP by thermal mixing [13] and that of a non-exponential SLR near the melting-point [11] suggest that (i) the local concentration of free radicals is (much) higher than expected, assuming a random distribution and (ii) the translational self-diffusion around the radicals is slower than in the pure medium, respectively. The combination of the two results seems to prove that, in this particular case, the presence of the radicals significantly modifies the host system, although this (important) modification cannot easily be inferred from the X-band E.P.R.…”

“…In this system, in addition to E.P.R. at both X and Q bands [9], dynamic nuclear polarization (DNP) [10][11][12][13], nuclear spin lattice relaxation via the radicals (SLR) [10][11] and electron-electron double resonance(ELDOR) [14,15], experiments have been performed. However, so far there is no picture which can explain consistently all these results.…”