1JCH couplings in Group 14/IVA tetramethyls from the gas-phase NMR and DFT structural study: a search for the best computational protocol

Abstract

Four tetramethyl compounds EMe₄ (E = C, Si, Ge, and Pb) were studied by high-resolution NMR spectroscopy in gaseous and liquid states at 300 K. Extrapolation of experimental vapor-phase C–H J-couplings to a zero-pressure limit permitted determining the ¹J_0,CHs in methyl groups of their nearly isolated molecules. Theoretical predictions of the latter NMR parameters were also performed in a locally dense basis sets/pseudopotential (Sn, Pb) approach, by applying a few DFT methods pre-selected in calculations of other gas-phase molecular properties of all these species and SnMe₄ (bond lengths, C–H stretching IR vibrations). A very good agreement theory vs. experiment was achieved with some computational protocols for all five systems. The trends observed in their geometry and associated coupling constants (¹J_CHs, ²J_HHs) are discussed and rationalized in terms of the substituent-induced rehybridization of the methyl group (treated as a ligand) carbon, by using Bent's rule and the newly proposed, theoretically derived values of the Mulliken electronegativity (χ) of related atoms and groups. All these χ data for the Group-14/IVA entities were under a lot of controversy for a very long time. As a result, the recommended χ values are semi-experimentally confirmed for the first time and only a small correction is suggested for χ(Ge) and χ(GeMe₃).