Solvent effect on the NMR chemical shieldings in water calculated by a combination of molecular dynamics and density functional theory

The solvent effect on the NMR chemical shielding in liquid water is calculated from a combination of molecular dynamics simulations and quantum chemical calculations For protons and O-17. The simulations are performed with three different potentials, ab initio as well as empirical ones, to study the influence of the force field. From the liquid configurations obtained in these simulations, molecules are randomly chosen together with neighbouring molecules to give clusters of water typical for the liquid at the selected temperature and density. Different cluster sizes are studied. The clusters are treated as supermolecules in quantum chemical calculations of chemical shifts by sumover-states density functional perturbation theory with individual gauge for localised orbitals. The influence of the quantum chemical method is studied with an ab initio coupled Hartree-Fock gauge including atomic orbitals calculations with different basis sets for a selected cluster. An average over clusters yields the chemical shielding in the liquid at the selected temperature and density. The calculated values for the gas-liquid shift, which are in best agreement with experiment, are -3.2ppm (exp. -4.26ppm) for the proton and -37.6ppm (exp. -36.1ppm) for O-17, but the results depend strongly on the chosen interatomic potential.