Semiempirical vs. Ab Initio Calculations of Molecular Properties. Part 3. Electric Field Gradients at 14N Nuclei in some Small Molecules

Electric field gradients (EFG) at nitrogen nuclei in some small and medium size molecules are calculated by the semiempirical CNDO/2Dand SCC-MO methods. A salient feature of our procedure is accurate evaluation of matrix elements of the EFG operator. Hence, comparison of EFG values with the results of more sophisticated ab initio procedures obtained by suitable basis sets indicates flaws and shortcomings of the semiernpirical schemes. It is concluded that EFGs provide a sensitive test of the anisotropy of the charge distribution of atoms in molecular environments. Therefore, they are invaluable for parametrization purposes leading to optimal screening constants of valence AOs to be used in semiempirical theories. As to the performance of the present CNDO/2D and SCC-MO methods, they provide only qualitative information about EFGs, the Iatter scheme being better because the CNDOj2D method fails to reproduce nuclear quadrupole coupling constants of nitriles