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The proton ENDOR spectra of various phenyl substituted hydrocarbon radicals in solution show a marked temperature dependence of the positions, widths, and amplitudes of particular lines. These effects are analyzed in terms of three relaxation mechanisms: (i) internal hindered rotation and (ii) torsional oscillations of the phenyl rings, and (iii) molecular tumbling. From the tempera-ture dependence of relaxation effects caused by mechanism (i) hindrance potentials for internal rotation of phenyl rings could be determined. The results are in good agreement with the pre-dictions of simple π-MO theory. Relaxation effects caused by (ii) turned out to be negligible. Mechanism (iii) produces strong variations in linewidths at low temperatures due to the different hyperfine anisotropics depending on nuclear position.

Electrolytically generated semiquinone radical ions have been studied in the nematic mesophase of p-azoxyanisole by EPR methods. The degree of molecular ordering of benzo-, naphtho-, and anthrasemiquinones varied from 0.085, 0.18 to 0.20, respectively. The observed shifts of the hfs splitting constants and the g factors are in reasonable agreement with theoretical predictions. The negative sign of the C1 13 splitting constant of benzosemiquinone has been confirmed.

In the nematic and isotropic phases of the liquid crystal p-azoxyanisole (PAA) the neutral radicals perinaphthenyl (PNT), triphenylmethyl (TPM), and pentaphenylcyclopentadienyl (PPCPD) have been studied by electron spin resonance techniques. A method is discussed for measuring the degree of ordering of the radicals by means of the observed g factors in the different phases. For PNT and TPM the components of the axially symmetric g tensor are determined and compared with the values predicted by STONE’S g factor theory. The shift of the hyperfine splitting constants (C13 and H1) of PNT and TPM agree rather well with the theoretical shifts calculated by the dipol-dipol interaction theory of Mc CONNELL and STRATHDEE. For TPM, however, the agreement is only satisfactory when all the long range contributions are considered. For PNT and TPM also the signs of the spin densities could be determined. PPCPD shows drastically that the validity of both g factor theory and hfs theory in nematic mesophases breaks down for unplanar molecules with large twist angles.

In the course of an investigation of electrolytically generated radical ions in the nematic mesophase of liquid crystals, we studied the influence of supporting electrolyte on the degree of molecular ordering. In the case of the radical anion of tetracyanoethylene (TCNE-) electrolytic generation results in a lower degree of ordering than chemical generation with sodium iodide. By using the neutral radical perinaphthenyl as a probe it was assured that the degree of ordering of the solvent is not disturbed by adding 0.1-m. tetrapropylammoniumperchlorate. A pronounced linewidth variation is observed for the chemically generated TCNE- which is attributed to spin density modulation by dynamic interaction with the Na+ counter ion.