Certain substituents seem to promote the insulating properties of the three molecular cores. We further assess substituents based on their electron withdrawing and donating properties. Such effects have been classified in the extensive physical organic chemistry literature. In Figure , the transmission at the Fermi energy is plotted for all substituted molecules sorted by their oftused Hammett constants s p , which are based on the substituent effect in para-substituted benzenes. Except for F and Cl, the substituent effect is fairly similar across the C222, Si222, and Ge222 series. However, there is no clear correlation across the full transmission data set when plotted against s p . Motivated by the strong transmission suppression seen for many of the carbon-based substituents, we separate the data set into two types of substituents: those based on carbon (blue in Figure ) and the remaining that are not carbon based (red in Figure ). We search further for quantitative correlations in substituent parameters that are determined in systems with suppressed or separated p-conjugation. In addition to s p , we plot the transmission values against the Hammett constant, s m , which are determined on meta-substituted benzenes and the inductive parameter F.