nJCHs in Group 14 tetramethyl species from gas-phase NMR and DFT calculations

What is it about?

The five tetramethyl EMe4 compounds (E = C, Si, Ge, Sn, and Pb) have been studied by high-resolution NMR spectroscopy in the gas state at 300 K. Extrapolation of experimental 1JCH couplings measured in the gas phase to the zero-pressure limit allowed the determination of these values in the methyl groups of the nearly isolated molecules. Theoretical predictions of these NMR parameters were also made in a locally dense basis set/pseudopotential approach (Sn, Pb) applying several DFT methods pre-selected in successful calculations of the other molecular properties in the gas phase, that is, the νas(CH3) C−H vibrational stretching fundamentals in infrared (IR) spectra of the five EMe4 species. This work can also be seen as a search for the best computational protocol to correlate various experimental data with their predicted values based on different theoretical approaches. Since all considerations involved ‘isolated’ molecules in the gas phase, the influence of the environment could be neglected, making all these calculations much easier and more reliable.

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Why is it important?

The trends observed in the geometry of the title tetramethyl systems and the associated coupling constants (1JCH and 2JHH) are discussed and rationalized in terms of substituent-induced rehybridization of the carbon atom of the methyl group (treated as a ligand), using Bent’s rule and newly proposed theoretically derived values of the Mulliken electronegativity values (XMs) of related atoms and -EMe3 groups. More precisely, they were explained by a gradual change in the s-character of the spn-hybridized carbon atoms that form their C-H bonds.

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