While all these parameters are determined empirically, F may provide a better measure of the substituent effect on the s-system as it uses a bicyclo[2.2.2]octane compound for reference. Shown in Figure , the transmission of the three cores with carbon-based substituents is plotted against the three different Hammett constants. While there is a weak correlation for C222, both Si222 and Ge222 transmission show strong correlation with all three empirical parameters. These correlations are consistently weaker for the non-carbon substituents as plotted in Figure . There is some correlation for these substituents in the Ge222 with s m and F, but in the remaining systems there is no correlation between the transmission and the Hammett constants.