Nuclear quantum effects in calculated NMR shieldings of benzene; a Feynmanpath integral study

The Feynman path integral Monte Carlo approach has been coupled to the gaug e including atomic orbital formalism in order to analyse the absolute magne tic shieldings of the benzene nuclei under the conditions of thermal equili brium. The Hamiltonian employed in the derivation of ensemble averaged NMR quantities is of the Hartree-Fock type. The basis set used is of 6-31G qual ity. The spatial delocalization of the atoms leads to a deshielding of both types of benzene nuclei relative to the shieldings experienced at the mini mum of the potential energy surface. This deshielding has to be traced back to bond length elongations in thermal equilibrium. The influence of the nu clear fluctuations on the NMR parameters of benzene is quantum driven up to temperatures of 400 K; classical fluctuations are of minor importance in t his low-temperature window.