Electron Paramagnetic Resonance (EPR) is a central spectroscopic technique for compounds with non-zero spin. The effective parameters from the EPR spin-Hamiltonian can today be calculated from rst principles using quantum chemical methods. We focus here on the hyperne coupling tensor, A, which is of great importance for deducing the structure of reaction intermediates with fast decay rates as for instance in metalloenzymatic transformations. We analyze calculations of the isotropic term, AKiso, in terms of molecular orbital contributions for a series of first row transition metal complexes and find that there is a great difference in the relative magnitude of contributions from frontier orbitals and inner or outer-core orbitals. Further analysis reveals that contributing frontier orbitals can be both ligand or metal d-orbital based while the core orbitals are predominantly of metal 2s or 3s character. Complexes where the frontier orbital contribution exceeds the core-orbital contributions are always small, ionic complexes (“class 1”). For these complexes the computational requirements with respect to the basis set are not severe and regular basis sets such as aug-cc-pVTZ provide reasonable results. Unfortunately both organometallic and traditional coordination complexes show a completely different behavior, where the core contributions to AKiso either are comparable (“class 2”) or far exceed (“class 3”) the contributions from the frontier orbitals. Agreement with experiment can for these complexes only be obtained by use of specialized core-property basis sets such as the aug-cc-pVTZ-J basis sets. Using the aug-cc-pVTZ-J basis set we nd that on overall, hybrid functionals perform best, although some exceptions are found. The exceptions are always “class” 2 or “class 3” types of complexes.
Journal of Chemical Theory and Computation, 2013, Vol 9, Issue 5, p. 2380-2388
EPR spectroscopy; Computational Chemistry; Quantum Chemistry; Transition metal complex; density functional theory; The Faculty of Science