Monte Carlo Simulation of the Effects of Molecular Motion on the EPR Spectrum of Nitroxide Free Radicals

We are concerned with the electron paramagnetic resonance (EPR) spectra of nitroxide free radicals in various molecular environments. These radicals may be described as a single electron interacting anisotropically with a nitrogen nucleus of spin 1 and an external magnetic field. We use exact and approximate solutions to the Hamiltonian to simulate spectra from oriented samples; we perform the orientation averages to obtain simulated polycrystalline spectra. We consider the effects of molecular motion on the system, and we develop a Monte Carlo scheme to evaluate the spectrum for a variety of tumbling rates. We compare the theoretical spectra so obtained with experimental spectra taken in solutions of varying rotational correlation time; excellent agreement is obtained. We estimate the rotational correlation time from the Monte Carlo parameters.