The carbon-based substituents we find here appear to be special for the silicon-and germaniumbased cores, but do not systematically promote interference effects in the carbon-based core. As previous studies have suggested, the s-interference effect is highly sensitive to several structural and electronic parameters. We have tested for correlations with structural parameters in our dataset but have not found any clear correlation, see Supporting Information part B for details. This indicates the effect of substituents is electronic (donor/acceptor effect) rather than from the distortion of the structure in molecular core. While these two effects (electronic and structural) are not independent of each other, the clear correlation we see for the carbon-based substituents with the Hammett constants suggests the effect is primarily electronic. Most likely, carbon-based substituents are special because they tune the electronic structure within an ideal range where there is s-interference in the transmission. This balance of parameters that enables interference effects in saturated molecules is still not well-described, but clearly the s-interference effect is very sensitive to both chemical and structural changes.