DFT Calculations of Isotropic Hyperfine Coupling Constants of Nitrogen Aromatic Radicals: The Challenge of Nitroxide Radicals

The performance of DFT methodology to predict with accuracy the isotropic hyperfine coupling constants (hfccs) of aromatic radicals containing N-14 nucleus is investigated by an extensive study in which 165 hfccs, belonging to 38 radical species, are obtained from calculations with B3LYP and PBE0 functionals combined with 6-31G*, N07D, TZVP, and EPR-III basis sets, and are compared to the reported experimental data. The results indicate that the selection of the basis set is of fundamental importance in the calculation of N-14 hfccs, whereas there is not so great an influence on the accurate computation of that parameter for H-1 nuclei. The values of the calculated N-14 coupling constants of aromatic nitroxide radicals using DFT methodology are noticeably lower than the experimental ones. A Very simple relation to predict these hfccs with high accuracy is proposed on the basis of the present results, as an interesting alternative to the highly computationally demanding integrated approaches so far used.