Ab initio MP2/GIAO/NBO study of the d-syn-axial effect in 13C NMR spectroscopy

The 13C chemical shifts of 20 rigid bicyclic compounds have been calculated with ab initio HF and MP2 methods. The calculations showed very good reproducibility of the experimental values. The molecular orbital interactions in the rigid, nearly planar d-syn-axial fragments in the isomeric groups of norbornane derivatives 1.x–4.x were studied in detail and were employed to explain the deshielding d-syn-axial effect in 13C NMR spectroscopy. q 2004 Elsevier B.V. All rights reserved.