Isotope Effects on Nuclear Magnetic Shielding in Molecular Hydrogen.

The secondary isotope shifts of six molecular hydrogen isotopologues (H2, D2, T2, HD, HT, and DT) were measured using gas-phase nuclear magnetic resonance spectroscopy. It was found that these isotope shifts are in satisfying agreement with performed ab initio quantum chemistry computations. However, there is a small systematic discrepancy between results of experiments and computations, i.e., the magnitudes of computed shifts are approximately 10% larger than those obtained from the experiments. This indicates that computations performed using the Born-Oppenheimer approximation are not sufficient for reproducing quantitatively the experimentally determined secondary isotope shifts of molecular hydrogen.