Chapter 6: Accurate Non-relativistic Calculations of NMR Shielding Constants
Published:09 Feb 2016
We present a brief description of the non-relativistic methods of quantum chemistry used to determine NMR shielding constants, with the focus on the accuracy of the available results. Following an outline of the theory underlying the calculation of NMR parameters we proceed to the discussion of the most important computational aspects: the choice of the basis set and the treatment of the electron correlation effects. Modifications of the standard atomic basis sets, leading to faster convergence of computed shielding constants, are described. In the analysis of the correlation effects we concentrate on the hierarchy of ab initio methods, proceeding from the Hartree–Fock approximation to the coupled cluster perturbation theory approach. In addition, we comment on the importance of the relativistic and vibrational corrections and the basic approaches used to incorporate them. The magnitude of different contributions is considered and the accuracy of the total shielding constants is analyzed. The selected illustrative results were obtained primarily for small molecular systems, making the discussed theoretical values suitable for direct comparison with experimental data from gas phase NMR spectroscopy.