Abstract :
Ab initio calculations of hydrogen NMR chemical shielding have been carried out at the correlation-including GIAO MP2 level on nine small molecules whose gas phase isotropic shieldings are well known. A range of basis sets from very small to rather large has been studied. While correlation effects at this level of theory are small, basis set and rovibrational effects are large. Large basis sets and the inclusion of rovibrational effects are necessary to calculate hydrogen shieldings well on an absolute basis (chemical shieldings), while the constraints are less demanding for calculations on a relative scale (chemical shifts) where agreement with experiment for the set of molecules studied as measured by the standard error is 0.11 ppm.
