Similar suppression and variation are seen in the ethyl, vinyl, and ethynyl substituted cases of Si222 and Ge222 (Figure and S4). Carbon-based substituents thus seem particularly promising for fine-tuning the insulating properties of Si222 and Ge222 systems. Although alkyl and phenyl substituents are quite benign from a chemical point of view, they are particularly promising because they are realistic synthetical targets for silicon and germanium based molecular cores. We hope these results will motivate studies of the properties of substituted cyclic and bicyclic silanes and germanes. The transmission suppression is less clear in the C222 systems than for the silanes and germanes. Shown in Figure , the transmission of C222-Me and C222-F does have antiresonances for some conformers. However, looking broadly at the C222 systems in Figure , it appears there are few cases with s-interference. We note the transmission of linear alkanes is much lower than that of linear silanes and germanes in general, and the suppression effect is thus quite small in C222. For example, the suppression of the transmission in C222-H is below an order of magnitude compared to C4-H, in good agreement with experimental results. Peralkylated bicyclo[2.2.2]octanes are not the most realistic synthetic targets, while fluorinated alkanes make for more realistic targets for single-molecule junction studies. The transmissions of all C222-F conformers are suppressed over a broad energy-range compared with C4-F. Of the substituents studied for C222, fluorinated bicyclo[2.2.2]octanes are thus an interesting prospect for organic interference-based singlemolecule insulators. Generally, however, they are still not as promising as the Si and Ge based systems as only the anti-conformer has a deep antiresonance near the Fermi energy.