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Quantum chemical calculation of NMR tensors
CSC newsflash (in finnish).
First principles quantum chemcical (QC) calculations of NMR parameters provide an independent supplement to experiment. The three objects to focus on are the nuclear shielding, spin-spin coupling and quadrupole coupling tensors. We use ab initio and density-functional techniques of electronic structure calculation for isolated species and model complexes.
The shielding tensor of any nucleus in a large polyatomic molecule can be reliably calculated using the gauge-including atomic orbital (GIAO) ansatz and different levels of approximation of the many-electron problem. While the uncorrelated Hartree-Fock method describes well saturated hydrocarbons, in cases of multiple bonds, inorganic molecules and noble gases it is essential to incorporate the effects of electron correlation. A stringent test to the methods is the spin-spin coupling tensor. This property necessitates a highly accurate description of the electronic structure of a molecule, as there are several different physical mechanisms that contribute. The spectra of nuclei with spin greater than one half are complicated by quadrupole coupling in anisotropic environments. Theoretically this involves the calculation of the electric field gradient tensor at nuclei.
The QC of NMR parameters requires very large computational facilities. We use both commercial and academic software installed at our own workstations as well as the computers of the CSC - IT Center of Science, Espoo, Finland.