Electric Field Dependence of EPR Hyperfine Coupling Constants

Area of Science:

• Physical Chemistry
• Quantum Chemistry
• Computational Spectroscopy

Background:

• The electric field dependence of the isotropic hyperfine coupling tensor (A_iso) for free radicals, known as the Bloembergen effect, is crucial for interpreting electron paramagnetic resonance (EPR) experiments.
• While studied for solid-state materials, this effect is less explored for isolated gaseous radicals.
• Accurate calculation of this dependence is essential for advancing spectroscopic analysis and understanding radical behavior.

Purpose of the Study:

• To introduce a simple, reliable, and computationally versatile method for calculating the electric field dependence of A_iso in free radicals.
• To establish a numerical approach that is independent of the specific quantum chemical method used for hyperfine tensor calculation.
• To validate the proposed method's performance across a diverse set of organic radicals.

Main Methods:

• The study employs numerical differentiation of the field-perturbed hyperfine coupling tensor (A) to determine its electric field dependence.
• This purely numerical technique ensures method independence, allowing flexibility in choosing quantum chemical approaches.
• Density functional theory (DFT) with CAM-B3LYP and coupled cluster with local pair natural orbitals (DLPNO-CCSD) were used to compute A_iso and its first derivatives (Bloembergen effect constants).

Main Results:

• The proposed numerical differentiation method successfully calculates the electric field dependence of A_iso for free radicals.
• The method was tested on 28 systems, including seven organic radicals, demonstrating its robustness and applicability.
• Calculations using CAM-B3LYP and DLPNO-CCSD provided accurate hyperfine tensors and their first derivatives.

Conclusions:

• A simple and reliable numerical method for calculating the electric field dependence of hyperfine coupling tensors in free radicals has been developed.
• This method, applicable to isolated gaseous radicals, enhances the analysis of EPR experiments.
• The findings provide a valuable tool for computational spectroscopy and the study of radical properties.