Relativistic effects on the shielding of SnH2XY and PbH2XY (X, Y = F, Cl, Br and I) heavy atom–containing molecules

JUAN I. MELO; ALEJANDRO MALDONADO; GUSTAVO A. AUCAR

THEORETICAL CHEMISTRY ACCOUNTS

SPRINGER

Año: 2010 p. 1 - 8

1432-881X

The relativistic polarization propagator approach is one of the most reliable methods availables today for the calculation of NMR spectroscopic parameters on heavy-atom containing molecules, though its implementation is still at RPA or FOPPA (first-order)level of approach. Two-component methods like the LR-ESC method makes possible the analysis of the electronic origin of relativistic effects due to its splitting in several mechanisms which are (or not) sensitive to the molecular structure or the nature of the chemicalenvironment of the atom under study. In this article we present the study of some nuclear magnetic shieldings on the heavy atom for the following systems: SnXH3 (X = H; F;Cl;Br; I), SnXY H2 (X; Y = F;Cl;Br; I) and PbXH3 (X = H; F;Br; I). Total LR-ESC calculations are confronted to benchmark RPA calculations and then analysed in order to get the main trends and discuss the electronicorigin of the shielding of two kind of atoms involved in such systems: central and substituent atoms.The electronic origin of the heavy atom effects on vicinal heavy atoms (HAVHA), recently proposed, is analysed. It is shown that the passive third order Spin orbit mechanism do not explain the total pattern though is still the most important. There are two othermechanisms involved: the so called here PSO-OZ and the L-PSO-K. Both mechanisms do contain the PSO perturbative Hamiltonian (which also include kinetic energy correcting terms). In the case of SnH2I2 the HAVHA effect on (Sn) is of the order of 16 %. Whenthe central atom is as heavy as Sn, the active SO contribution on the shielding of such atom becomes larger than the passive SO, which is small in this case. This would mean that the HALA-type effect is strongly diminished when applied on a vicinal heavy atom.Quite a similar pattern though with larger relativistic effects are observed for the central  lead atom.