Since nitroxide free radicals are spin labelled molecules, we use their resonance spectra to get information about their crystallographic structure. ESR spectra of nitroxide single crystals (pyrroline, piperidine, and pyrrolidine) show only a single line. From the evolution of line position with crystal orientation we try to obtain information about the orientations of g tensors. Generally, we calculate the angles defining individual cc > NO groups orientations with an accuracy better than 5 deg. Principal g values are given for several nitroxides.
The structure of the ferroelectric-ferroelastic phase of 2,2,6,6-tetramethylpiperidine 1-oxyl (C9HlsNO, 'tanane') was refined from neutron diffraction data [a = 22.64 (2), b = 21.71 (2), c = 8.030 (6) ,/k, Faa2, z = 16, 1022 observed reflections, 262 parameters, R w = 0.06]. The crystal was kept as a single domain by a uniaxial constraint. Little structural change was observed with respect to the tetragonal disordered hightemperature phase. Results of empirical calculations of electrostatic and van der Waals intermoleeular energies for the three forms of tanane are in agreement with their domains of stability.
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