Invited lecture: Electronic origin of diamagnetic contribution to NMR shielding

GUSTAVO A. AUCAR

Ottrott

Congreso; REHE Relativistic Effects on Heavy Elements-2007; 2007

Between that molecular properties for which relativistic effects are very important the NMR spectroscopic parameters are perhaps the most studied. In the case of nuclear magnetic shielding, sigma, this is because the effective external magnetic field that interacts with thenuclear spin depends on the shielding of the whole electronic system.From a theoretical and non relativistic point of view, the diamagnetic contribution to sigma do arise from a term in the Interaction Hamiltonian that depends on A2, the square of the vector potential. When going to a more realistic ( or relativistic) domain, such a term does not appear.It is now well-known that diamagnetic contributions to sigma comes from the negative-energy electronic states.In this lecture starting from a QED-based formulation of NMR-sigma (that gives solid grounds to the relativistic polarization propagator theory), I will show the appearance of a closed branch of negative-energy electronic states that gives the whole contribution to the diamagnetic part of sigma. I will also present the pattern of contributions of that branch of energy states depending on: i) Z, ii) the molecular system and iii) the occupied electronic states. I will finally give a possible explanation for that unexpected behavior that is completely molecular-system-independent.