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 ca lculated from a combination of molecular dynamics simulations and quan tum chemical calculations For protons and O-17. The simulations are pe rformed with three different potentials, ab initio as well as empirica l ones, to study the influence of the force field. From the liquid con figurations obtained in these simulations, molecules are randomly chos en together with neighbouring molecules to give clusters of water typi cal 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 local ised orbitals. The influence of the quantum chemical method is studied with an ab initio coupled Hartree-Fock gauge including atomic orbital s calculations with different basis sets for a selected cluster. An av erage over clusters yields the chemical shielding in the liquid at the selected temperature and density. The calculated values for the gas-l iquid 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.