Recent Developments in Absolute Shielding Scales for NMR Spectroscopy

AUCAR, I. AGUSTÍN; AUCAR, GUSTAVO A.

Annual Reports on NMR Spectroscopy

Academic Press

Año: 2019; p. 77 - 141

The absolute value of the nuclear magnetic shielding tensor, σ, cannot be directly mea-sured as such, though at the moment experimental devices are available to obtain themby extrapolation to zero-density limit. This is the reason why since the beginning of NMRspectroscopy to obtain the experimental values of this parameter required the interplaybetween theory and experiments.Early this century it was experimentally probed that the Ramsey?Flygare model,largely used to obtain the most accurate semiexperimental values of shieldings, doesnot work well when heavy-atom containing molecules are considered. Then, the searchfor a relativistic relationship like Ramsey?Flygare became highly necessary. This searchhas taken some years for being successful.Within the research programme that focused on such a goal, the first importantbreakthrough was the finding of the relativistic Hamiltonian for spin?rotations interac-tions. Once this crucial step was overcome it was possible to develop some relativisticmodels, being the most accurate the so-called M-V. We shall show here some of themost important steps that were given in the search for a relativistic relationshipbetween σ and the spin?rotation tensor, M. We will describe the bottom-up procedurethat make it possible, together with relativistic (four-component) theories of shieldingsand spin?rotations that were published recently.Why it was important to find that new relationship? There are now experimentaldevelopments that permits direct measurements of absolute shieldings in gas-phaseNMR spectroscopy, and so, to compare with. It is also possible to accurately calculatethem by using four-component methodologies, from which very small effects likenuclear charge distribution, Breit/Gaunt electron?electron interactions and QED effectsare included. All of them will be highlighted here in some detail.Lastly we shall stress the new understandings that arise from these developmentstogether with some new challenges.