C13–H and X—H Coupling Constants for Compounds of the Type X(CH3)4

From high-resolution proton magnetic resonance experiments, absolute magnitudes of JC13–H and JX–C–H have been determined for compounds of the type X(CH3)4, where X=Si, Ge, Sn, and Pb. The JC13–H values yield the s characters of the carbon orbitals in the C–H and C—X bonds as well as the Juan—Gutowsky s character affinity ΔX. Rough calculations of JX–C–H based on the Fermi contact term give widely varying results depending on the value of the effective charge used. A straight line correlation of JX–C–H with n3 can be explained crudely on the basis of the Fermi contact term.