The C(s) conformation of the tert-butyl cation 3 was established to be the preferred global energy minimum using a combination of ab initio, DFT, and CCSD(T) methodology with correlation-consistent basis sets. The potential energy surface of methyl rotation involving the C(3v), C(s), and C(3h) forms, however, in accord with previous studies, is quite flat. The computed IR absorptions of 3 indicate that it has the greatest degree of electron donation from C-H bonds into the C(+)-C bonds. The experimental (13)C NMR chemical shifts also agree very well with the experimental data.

The C(3h) conformation of the trimethylsilicenium ion 1 was established to be the preferred global energy minimum structure based on energy calculations. Because C-H hyperconjugation occurs least favorably in this conformation of the analogous tert-butyl cation, it may not contribute in large part to the stabilization of this cation, especially given the ineffectiveness of the 3p-2sp(3) overlap that would need to be involved. This is in contrast with the preferred C(s) global energy conformers of the tert-butyl cation. The C(2v) structure 4 and C(2) enantiomers 6 and 7 are the preferred conformations of the dimethylsilicenium ion based on energy comparison. None of these structures have C-H bonds ideally oriented for hyperconjugation with the empty p orbital of the cationic silicon, indicating that it does not likely stabilize the ion to any significant extent. The computed IR spectra and (29)Si, (13)C, and (1)H NMR chemical shifts of the isomers were also discussed. Whereas the studied alkylsilicenium ions are thermodynamically stable, their observation as persistent ions in solution is much more difficult because of their kinetic instability toward varied electron-donating solvents.

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