Absolute value of the nuclear magnetic shielding of silicon and germanium atoms in Si/Ge(CH3)4

Abstract

The reference value of NMR magnetic shieldings, σref(X), of a nucleus X is of highest importance when theoretical analysis of chemical shifts are envisaged. For light atoms the absolute scale can be obtained using an old relationship among spin–rotation constants and magnetic shieldings, though for heavy-atom-containing molecules such nonrelativistic relationship is not valid any longer. Then, for such molecules the search for σref needs new strategies to be followed. We present here new values of σref(Si) and σref(Ge) for tetramethyl silane and tetramethyl germanium that were obtained applying a simple procedure which mix accurate experimental chemical shifts and theoretically obtained magnetic shieldings in a set of selected molecular systems. We considered that in experiments one usually measures chemical shifts, (δ). We calculated σref(Si) and σref(Ge) from a set of seventeen and sixteen heavy-halogen-containing molecules, respectively. We found out that σref[Si; Si(CH3)4] in gas phase should be close to 410.49±6.77ppm and σref[Ge; Ge(CH3)4] should be close to 1705.29±19.51ppm. Such theoretical values were obtained by performing calculations within the relativistic polarization propagator method, RelPPA, at RPA level of approach. Based on those values of σref, we recalculated the chemical shifts of the whole set of molecular systems obtaining a good reproduction of known experimental results. We also obtained a highly correlated relationship among the absolute values of the nuclear magnetic shieldings of Si and Ge atoms in halogen substituted tetrahedral compounds.