Nuclear quantum effects in calculated NMR shieldings of ethylene; a Feynman path integral – ab initio study

The Feynman path integral Monte-Carlo formalism has been combined with the gauge-including atomic orbital (GIAO) approach to study the absolute magnetic shieldings of C2H4 under consideration of the thermal and quantum degrees of freedom of the nuclei. An ab initio Hamiltonian has been employed for the statistical averaging of NMR parameters. The spatial fluctuations of the atoms around their equilibrium positions lead to a deshielding of both types of nuclei relative to their shieldings at the minimum of the potential energy surface. This behavior is caused by quantum mechanical effects. It is supported by bondlength elongations in thermal equilibrium.