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 magnet ic shieldings of C2H4 under consideration of the thermal and quantum degree s of freedom of the nuclei. An ab initio Hamiltonian has been employed for the statistical averaging of NMR parameters. The spatial fluctuations of th e atoms around their equilibrium positions lead to a deshielding of both ty pes of nuclei relative to their shieldings at the minimum of the potential energy surface. This behavior is caused by quantum mechanical effects. It i s supported by bondlength elongations in thermal equilibrium. (C) 2000 Else vier Science B.V. All rights reserved.