ON THE CALCULATION OF HYDROGEN NMR CHEMICAL SHIELDING

Ab initio calculations of hydrogen NMR chemical shielding have been ca rried out at the correlation-including GIAO MP2 level on nine small mo lecules whose gas phase isotropic shieldings are well known. A range o f basis sets from very small to rather large has been studied. While c orrelation effects at this level of theory are small, basis set and ro vibrational effects are large. Large basis sets and the inclusion of r ovibrational effects are necessary to calculate hydrogen shieldings we ll on an absolute basis (chemical shieldings), while the constraints a re less demanding for calculations on a relative scale (chemical shift s) where agreement with experiment for the set of molecules studied as measured by the standard error is 0.11 ppm.