Approximate methods for the fast computation of EPR and ST-EPR spectra. III. Extension of the perturbation approximation to conditions of overmodulatio

The electron paramagnetic resonance (EPR) and saturation transfer electron paramagnetic resonance (ST-EPR) spectra typical of nitroxide spin labels obeying an isotropic, brownian diffusion model are theoretically investigated. The derivative approximation discussed in the first paper of this series (I) is examined and generalized to consider large modulation amplitudes. The approximation considers both molecular and applied modulation effects with the Zeeman modulation frequency acting as a perturbation parameter. The application of the approximation to first and second harmonic simulations is discussed. The observations of I are preserved for first harmonic spectra. The approximation is able to accurately reproduce the in-phase absorption and in-phase-quadrature dispersion signals. In-phase dispersion signals are reproducible at low microwave powers while in-phase-quadrature signals may be reproduced at higher powers. The approximation is of limited utility for second harmonic simulations and is particularly unable to reproduce the in-phase-quadrature absorption signal at this harmonic. The approximation enables a marked reduction in both the core and CPU time required for simulations. Factors of 3 and 4.2 respectively were realized in the simulations of slow isotropic brownian diffusion.