Solvent effects on NMR isotropic shielding constants.: A comparison between explicit polarizable discrete and continuum approaches

The gas-to-aqueous solution shifts of the O-17 and C-13 NMR isotropic shielding constants for the carbonyl chromophore in formaldehyde and acetone are investigated. For the condensed-phase problem, we use the hybrid density functional theory/molecular mechanics approach in combination with a statistical averaging over an appropriate number of solute-solvent configurations extracted from classical molecular dynamics simulations. The PBE0 exchange-correlation functional and the 6-311++G(2d,2p) basis set are used for the calculation of the shielding constants. London atomic orbitals are employed to ensure gauge-origin independent results. The effects of the bulk solvent molecules are found to be crucial in order to calculate accurate solvation shifts of the shielding constants. Very good agreement between the computed and experimental solvation shifts is obtained for the shielding constants of acetone when a polarizable water potential is used. Supermolecular results based on geometry-optimized molecular structures are presented. We also compare the results obtained with the polarizable continuum model to the results obtained using explicit MM molecules to model the bulk solvent effect.