Relativistic and electron correlation effects on the NMR shieldings of molecules containing Sn and Pb atoms

A. F. MALDONADO; G. A. AUCAR

JOURNAL OF PHYSICAL CHEMISTRY A

AMER CHEMICAL SOC

Lugar: Washington; Año: 2014 vol. 118 p. 7863 - 7875

1089-5639

The reference values for NMR magnetic shieldings, sigma ref, are of highest importance when theoretical analysis of chemical shifts are envisaged. The fact that the non relativistically valid relationship among spin-rotation constants and magnetic shieldings is not any longer valid for heavy atoms makes that the search for sigma ref for such atoms needs new strategies to follow. We present here results of sigma ref obtained applying a recent procedure which mix accurate experimental chemical shifts (delta) and theoretical magnetic shieldings (sigma). We calculated the absolute values of sigma (Sn) and sigma (Pb) in a family of heavy-halogen containing molecules. We found out that sigma ref (Sn; Sn(CH3 )4 ) in gas phase should be close to 3864.11 ± 20.05 ppm (0.5 %). In the case of Pb, sigma ref (Pb; Pb(CH3 )4 ) should be close to 14475.1 ppm ± 500.7. Such theoretical values correspond to calculations with the relativistic polarization propagator method, RelPPA, at RPA level. They are closer to experimental values as compared to those obtained with few different functionals like PBE0, B3LYP and BLYP. We studied tin and lead shieldings of the XY4-nZn (X = Sn, Pb; Y , Z = H, F, Cl, Br, I) and PbH4-nIn (n = 0, 1, 2, 3, 4) family of compounds with 4-component functionals as implemented in the DIRAC code. We found that results of calculations with RelPPA-RPA are more reliable than the DFT ones. We argue on why those DFT functionals must be modified in order to obtain better results of NMR magnetic shieldings within the relativistic regime. There is a dependence among both, electron correlation and relativistic effects that should be introduced in one way or another in the functionals. The functionals were parameterized only within a nonrelativistic regime and so they are not able to properly introduce relativistic effects. We finally show that in the analysis of magnetic shieldings for the above mentioned family of compounds, one should consider the newest and so called heavy-atom effect on vicinal heavy atoms, HAVHA. Such effects are among the most important relativistic effects in the above mentioned family of compounds.