ROVIBRATIONAL EFFECTS, TEMPERATURE-DEPENDENCE, AND ISOTOPE EFFECTS ONTHE NUCLEAR SHIELDING TENSORS OF WATER - A NEW O-17 ABSOLUTE SHIELDING SCALE

We calculate the rovibrational corrections, temperature dependence and isotope shifts of the isotropic and anisotropic nuclear shieldings of the water molecule. This is the first correlated study of rovibration al effects on the nuclear shieldings in the water molecule and the fir st study of these effects on the shielding anisotropies. The use of a large restricted active space self-consistent field wave function and a large basis set ensures that the results are of high accuracy. The r ovibrational corrections are found to be substantial, 3.7% and 1.8% fo r the isotropic oxygen and hydrogen shieldings, respectively, in the ( H2O)-H-1-O-17 isotopomer at 300 K. For the shielding anisotropies and asymmetry parameters, the corresponding relative corrections are even larger. The changes in the shielding tensors due to molecular rotation and vibration are of the same order of magnitude as-and in some cases even exceed-the changes due to electron correlation. The accuracy of our calculated rovibrationally corrected oxygen shielding leads us to propose a new absolute shielding scale for the O-17 nucleus: 324.0 +/- 1.5 ppm for the O-17 shielding of (H2O)-H-1-O-17 in the gas phase (30 0 K). This shielding scale is supported by recent high-level CCSD(T) c alculations on carbon monoxide. The absolute oxygen shielding constant s of some oxygen-containing molecules are calculated using the new shi elding scale and experimental data on oxygen chemical shifts, comparin g the absolute shieldings to the results of recent high-level theoreti cal calculations. (C) 1998 American Institute of Physics. [S0021-9606( 98)30143-9].