The interpretation of molecular magnetic hyperfine interactions

Investigations of the hyperfine structure in the excited electronic states of several free radical species have revealed shortcomings in the currently accepted values used for the theoretical interpretation of such interactions. We introduce updated reference atomic values from a combination of experimental observations and ab initio calculations. The latter are at Hartree-Fock and multireference configuration interaction levels of theory and several atomic test cases are discussed. Furthermore, ground and excited electronic state hyperfine coupling constants are calculated using both levels of theory for a range of first- and second-row diatomic hydride and nonhydride radicals. These results, together with a selection of other experimental measurements are then compared with experimental data where available, and the implications of the revised interpretation are discussed.