Four component calculation of Nuclear magnetic shieldings for Interhalogen molecules. Spin orbit and spin free relativistic effects.

SERGIO S. GOMEZ AND RAMIRO ARRATIA PEREZ

Punta Tralca, Chile

Workshop; 5th Workshop of Computational Chemistry and Molecular Spectroscopy; 2006

Universidad de Andres Bello. Santiago, Chile.

There is an increasing interest in understanding the electronic origin of relativistic effects in nuclear magnetic shielding tensor of the NMR spectra. The Heavy Atom on Light Atom(HALA) effect, which is the relativistic modification of  the shielding of the light atom due to the heavy atom in the molecule, can be explained trough the Spin Orbit mechanism(SO). On the other hand there is a relativistic effect on the Heavy Atom due to the Heavy Atom(HAHA) itself, whose origin was proposed recently in terms of the paramagnetic Mass Velocity External Field(MVEF) mechanism. Previous calculations of this terms in molecules containing Hydrogen and one heavy atom shows that indeed this mechanism account for more than 90% of the relativistic corrections of the paramagnetic part of   shielding at the heavy atom, regardless the structure of the molecule(2,3). However, in this presentation we show trough ab-initio four components calculations with FX(X=F,Cl,Br,I), how the inclusion of Fluoride instead of Hydrogen in the molecule enhances the Spin Orbit contribution to a value of the order of MVEF term. This results suggest that the interpretation of HAHA e ect as MVEF mechanism should be taken carefully.