Electron correlation and basis set effects on the 17O and 1H nuclear magnetic shieldings in water clusters (H2O)n (n=2-5)

The relative importance of electron correlation and basis set effects on the calculated O-17 and H-1 NMR isotropic shieldings and shielding anisotropies in a series of small water clusters including between two and five water molecules have been studied by means of the Hartree-Fock (HF) and second-order Moller-Plesset (MP2) approaches utilizing gauge-including atomic orbitals (GIAOs),within the 6-31G(d,p), 6-31++G and 6-311++G(2d,2p) basis sets. The results indicate that the inclusion of correlation effects and use of diffuse functions is essential in the case of hydrogen-bonded O-17 nuclei, while hydrogen-bonded protons are already well-described at the HF-GIAO level of theory within basis sets including polarization functions. It has been shown that while the basis set superposition error (BSSE) effects on the isotropic shieldings and shielding anisotropies of hydrogen-bonded protons are safe to neglect, in the case of hydrogen-bonded oxygens these have to be dealt with through the use of an appropriate larger basis. (C) 2002 Elsevier Science B.V. All rights reserved.