Near-DFT: Accurate Prediction of Isotropic Hyperfine Couplings in Nitrogen Radicals

Accurate theoretical prediction of EPR (electron paramagnetic resonance) spectra is crucial for elucidating reaction mechanisms, identifying unknown radicals, and guiding chemical transformations. This study introduces near-DFT (nuclear electron spin density approximation via radial functions for DFT), a novel computational approach that synthesizes two fundamental physical principles into a new predictive framework for the isotropic hyperfine coupling constant (A_iso). First, it systematically applies the Kato cusp condition to correctly describe electron spin density near the nucleus. Second, a pseudo-π orbital model, constrained by this condition, reveals that NLP-type (type of nitrogen-centered radical without a lone pair) mononitrogen radicals exhibit a significant SOP (spin orbit polarization) effect compared to the predominantly SP (spin polarization) effect for LP-type (type of nitrogen-centered radical with a lone pair) radicals. Building upon these insights, near-DFT demonstrates high accuracy in predicting EPR parameters (within 5% of experimental values) for both mononitrogen and complex polynitrogen radicals, offering a robust and efficient tool for the precise interpretation of EPR spectra. This method also saves significant calculation time compared to CCSD.