Theoretical studies of magnetic shielding in H2O and (H2O)2

Abstract

Magnetic shielding constants are calculated for the nuclei in H₂O and (H₂O)₂ using a slightly extended$set of atomic functions modified by gauge factors. Calculated shielding results for the theoretical equilibrium structures of H₂O and (H₂O)₂ lead to a value for the hydrogen bond shift of −2.95 ppm, which is in reasonable agreement with the available experimental data. The anisotropy of the magnetic shielding tensor associated with the hydrogen bonded proton is found to increase by 12 ppm on dimer fromation. This suggestes that proton magnetic shielding anisotropies may be more sensitive to features of hydrogen bonding than are isotropic values. An analysis of the theoretical results reveals that the downfield hydrogen bond shift arises (a) from a decrease of the electronic charge on the hydrogen bonded proton, and (b) from deshielding effects of the currents induced on the oxygen atom of the porton acceptor by the external magnetic field. For (H₂O)₂ these two types of contribution are of equal importance. The latter type of contribution (b) is found to be almost tottally responsible for the anisotropy changes produced on dimer formation. The sensitivity of the shielding tensor associated with the hydrogen bonded proton to changes in the intermolecular geometrical parameters of (H₂O)₂ is examined. The calculatted isotropic and anisotropic values of thius tensor show large changes of the O⋅⋅⋅O distance%is reduced below 3.5 Å. In contrast, the sensitivity of this shielding tensor to the relative orientation of the water monomers in (H₂O)₂ is quite small. Finally, the results of relaxing the geometry of the proton donor are reported. The isotropic value of the shielding at the hydrogen bonding proton is found to decrease markedly as the O−H distance of O−H⋅⋅⋅O is increased slightly from its equilibrium value.