Modeling EPR parameters of nitrogen containing conjugated radical cations

Abstract

The accuracy of DFT methodology on modeling isotropic hyperfine coupling constants (hfccs or a_iso) of conjugated radical cations containing ¹⁴N nucleus, intermediates involved in many biological and chemical processes, is investigated. A set of 222 hfccs, belonging to 50 radical species, are computed at the following levels of theory: PBE0/N07D, B3LYP/6-31G*, B3LYP/N07D, B3LYP/TZVP, and B3LYP/EPR-III, and compared to the available experimental values. In general, these five combinations of methods and basis sets estimate a_iso(¹H) in good agreement with experimental data. Conversely, selection of the basis set is of fundamental importance for accurate prediction of the ¹⁴N hfccs. A thorough analysis of the computed data allows us to establish that the best functional/basis set combination for obtaining accurate nitrogen coupling constants in conjugated radical cations is PBE0/N07D//B3LYP/6-31G*, which provides values with a discrepancy smaller than 1.5 G for most of constants. The model has been successfully tested on the calculation of ¹⁴N hfccs for different radical cation derivatives of syn-1,6:8,13-diimino[14]annulene, which is a highly demanding test because the strong experimental change in the a_iso(¹⁴N) values critically depends on the local geometry of the diimino fragment which is function of the polymethylene chain length. Furthermore, a whole analysis of the computed structural parameters, NBO and distribution of the spin density of these annulene radicals has allowed to explain the huge increase of the a_iso(¹⁴N) as well as their stability just for certain polymethylene chain length evidencing the formation of a N–N three electron σ bond.