NMR SHIELDING CALCULATIONS INCLUDING ELECTRON CORRELATION - BENZENE, PYRIDINE AND THE N-AZINES

Ab initio calculations using the second-order correlated localized orbital-local origin method (SOLO) are presented for the nuclear shielding tensors of N-15 and C-13 nuclei in the series of 6-membered aromatic heterocycles from pyridine to s-tetrazine, and for C-13 in benzene. Correlation effects consistently increase the isotropic shieldings, for N-15 by up to 50 ppm and for C-13 by up to 10 ppm, and lead to significantly improved agreement with experiment relative to calculations al the Hartree-Fock (LORG) level for the strongly deshielded N-15 nuclei in this series. For the principal-axis shieldings second-order correlation is found consistently to decrease the anisotropy and the width of the shielding pattern.