Computation Of Isotropic Hyperfine Coupling Constants.pdf

The isotropic hyperfine coupling constant (hcc) is a crucial parameter in understanding the magnetic properties of paramagnetic molecules. The hcc is determined by the electron density at each nucleus with a nonzero magnetic moment, and its accurate calculation requires a combination of a suitable basis set, a molecular quantum mechanical method, and a method to calculate the electron density. The unrestricted formalism used in density functional theory (DFT) can lead to spin contamination, which affects the quality of the computed hcc values. A new approach based on the first-order breathing orbital self-consistent field (FOBO-SCF) method has been developed to overcome this limitation. The FOBO-SCF method is an almost parameter-free wave function method that can accurately treat spin delocalization and spin polarization effects while avoiding spin contamination. The method has been tested on a series of small radicals, including nitroxide radicals, and compared to high-level ab initio methods. The results show very encouraging agreement, and the method has been applied to compute the hcc of a challenging system, the DEPMPO-OOH radical in various conformations. The reference values obtained on this large system validate a cheap computational method based on DFT. The FOBO-SCF method also allows for the rationalization of the results according to a relatively simple scheme based on a two-step mechanism, involving spin delocalization and spin polarization. The method has been applied to study the dependence of the hcc values on the nitrogen pyramidalization degree of freedom for four small nitroxide radicals, and the results are compared to QCISD level of theory. The very good agreement with QCISD reference values is interpreted in light of the spin delocalization and spin polarization mechanisms. The FOBO-SCF method has been used to study the DEPMPO-OOH radical resulting from the trapping of the superoxide radical by the DEPMPO nitrone, and the corresponding complex ESR spectrum is usually interpreted using a model spin Hamiltonian featuring couplings with 1H, 14N, and 31P nuclei for different DEPMPO-OOH isomers/conformers in chemical exchange. The theoretical determination of the DEPMPO-OOH hcc values is a challenging task due to the dimensions of this system, but the present study enables the provision of reliable reference hcc values on this realistic system, which eventually validate a much cheaper computational approach based on DFT.