Electric Field Dependence of EPR Hyperfine Coupling Constants.

In this article, we introduce a simple but reliable method to calculate the electric field dependence of the isotropic hyperfine coupling tensor for free radicals. This dependence, also referred to as the Bloembergen effect, can be of interest in analyzing EPR experiments for solid-state materials but is rarely studied for isolated radicals in the gaseous phase. The proposed method uses the numerical differentiation of the field-perturbed tensor and, consequently, as a purely numerical method, does not depend on a quantum chemical method used to determine the hyperfine tensor . To test the performance of the proposed method, we used a set of 28 systems, including seven organic radicals, the majority of which were taken from a recent benchmark study by Bartlett's group. We employed the well-tested and robust density functional theory (DFT) functional, namely CAM-B3LYP, and the variant of the CCSD method based on local pair natural orbitals, namely DLPNO-CCSD to calculate the tensor itself, and all first derivatives or Bloembergen effect constants .