Nuclear degrees of freedom in calculated isotropic hyperfine coupling constants of the ethyl radical: a Feynman path integral – ab initio study

Abstract The Feynman path integral quantum Monte Carlo formalism has been combined with an electronic Hamiltonian of the B3LYP-EPRIII type to study the isotropic hyperfine coupling (hfs) constants of the ethyl radical in thermal equilibrium ( T =25 K). The nuclear vibrations and the intramolecular rotation around the C–C bond have a sizeable influence on the hfs parameters. The thermally averaged numbers differ from single-nuclear configuration values calculated at the minimum of the potential energy surface. The analysis of the distribution functions for the isotropic hfs constants of C 2 H 5 offers insight into the strong coupling between calculated hfs parameters and the nuclear configuration adopted.