1JCHs in Group 14/IVA tetramethyl species from gas-phase NMR

What is it about?

Four tetramethyl EMe4 compounds (E = C, Si, Ge, and Pb) have been studied by high-resolution NMR spectroscopy in the gas state at 300 K. Extrapolation of experimental 1JCH couplings 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 (Sn, Pb) approach by applying several DFT methods pre-selected before in successful calculations of other molecular properties in the gas phase, that is the νas(CH3) C−H vibrational stretching fundamentals in IR spectra of all 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, which greatly facilitated the calculations.

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Why is it important?

The trends observed in the geometry of the title systems and associated coupling constants (1JCH, 2JHH) are discussed and rationalized in terms of the substituent-induced rehybridization of the carbon atom of the methyl group (treated as a ligand), by using Bent’s rule and the 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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