Resonance Spectra of a Paramagnetic Probe Dissolved in a Viscous Medium

Abstract

A model is presented for calculating the paramagnetic resonance (EPR) spectrum of vanadyl acetylacetonate (VAAC) dissolved in either a liquid crystal or isotropic solvent. It employs density matrix formulation in the rotating reference frame. The molecules occupy several discrete angles with respect to the magnetic field and can relax to neighboring positions in a characteristic time tau(theta). The form of tau(theta) is found from a diffusion approach, and the magnitude of tau(theta) is a measure of how freely the VAAC probe tumbles in the solvent. Spectra are predicted for values of tau between 10 to the minus 11th power sec and 10 to the minus 7th power sec. The EPR spectrum, in the isotropic case, is obtained be summing the contributions from the allowed angles weighted by the polar volume element, sin theta. When applying the model to the nematic liquid crystal case it is also necessary to multiply by the Saupe distribution function. For this case tau(theta) is obtained from the diffusion approach in which two diffusion constants are employed to reflect the difference in the parallel and perpendicular components of the viscosity.