Electron spin resonance of symmetrical three-spin systems in nematic liquid crystals

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Abstract
The Alexander-Kaplan method is applied to analyse the E.S.R. spectra of symmetrical three-spin systems, undergoing fast, intermediate and slow rotational diffusion in nematic liquid crystals. Particular emphasis is given to the slow-motional region. The line shapes are calculated from a spin hamiltonian, which considers Zeeman, exchange, dipolar and hyperfine interactions of the three unpaired electrons. The predictions of the theory are tested by comparison with the temperature-dependent E.S.R. spectra of trisverdazyl radicals dissolved in nematic phase V. Good agreement is found between experimental and simulated spectra. A detailed analysis provides values for the rotational correlation times and order parameters of the radicals. They are related to the viscosity and order of the pure mesophase. A comparison shows that three-spin systems are sensitive and reliable spin probes for the investigation of viscous liquid crystals.