Recent Developments in Absolute Shielding Scales for NMR Spectroscopy

Abstract

The absolute value of the nuclear magnetic shielding tensor, σ, cannot be directly measured as such, though at the moment experimental devices are available to obtain them by extrapolation to zero-density limit. This is the reason why since the beginning of NMR spectroscopy to obtain the experimental values of this parameter required the interplay betweentheory and experiments.Early this century it was experimentally probed that the Ramsey-Flygare model, largely used to obtain the most accurate semi-experimental values of shieldings, does not work well when heavy-atom containing molecules are considered. Then, the search for a relativistic relationship like Ramsey-Flygare became highly necessary. This search has taken some yearsfor being successful.Within the research program that focused on such a goal, the first important breakthrough was the finding of the relativistic Hamiltonian for spin-rotations interactions. Once this crucial step was overcome it was possible to develop some relativistic models, being the most accurate the so called M-V. We shall show here some of the most important stepsthat were given in the search for a relativistic relationship between σ and the spin-rotation tensor, M . We will describe the bottom-up procedure that make it possible, together with relativistic (four-component) theories of shieldings and spin-rotations that were published recently.Why it was important to find that new relationship? There are now experimental developments that permits direct measurements of absolute shieldings in gas-phase NMR spectroscopy, and so, to compare with. It is also possible to accurately calculate them by using four-component methodologies, from which very small effects like nuclear charge distribu-tion, Breit/Gaunt electron-electron interactions and QED effects are included. All of them will be highlighted in some detail.We at least shall stress the new understandings that arise from these developments together with some new challenges.