Approximate methods for the fast computations of EPR and ST-EPR spectra. IV. A perturbation approach with back substitution of the diagonal density matrix elements

Abstract

The electron paramagentic resonance (EPR) and saturation transfer electron paramagnetic resonance (ST-EPR) spectra of nitroxide spin labels are theoretically investigated. The perturbation treatment, employing the Zeeman modulation frequency as the perturbation parameter described in papers I and III of this series is modified to incorporate back substitution of the diagonal density matrix elements. The calculations are extended to consider large Zeeman modulation amplitudes and frequencies employed in recent ST-EPR experiments. In every case the accuracy of simulation of the experimentally important first harmonic in-phase absorption, first harmonic phase-quadrature dispersion, and harmonic phase-quadrature absorption signal components is enhanced when back substitution is employed. Moreover, the accuracy of simulation of most signal components is employed for the case of both high Zeeman modulation frequencies and large modulation amplitudes.