Using the MP2, CCSD(T), and DFT (B3LYP) methods, the structures and energies of the 2-butyl cation (C4H9+) were calculated. Energetically, the C–C hyperconjugated structure 1 and hydrogen-bridged structure 2 were found to be almost identical at all levels. The 13C NMR chemical shifts of 1 and 2 were computed by the GIAO-CCSD(T) method using different geometries. On the basis of calculated relative energies and calculated 13C NMR chemical shifts, an equilibrium involving 1 and 2 (in a 50:50 ratio) seemed likely responsible for the experimentally observed 13C NMR chemical shifts in superacid solutions at −80 °C. However, on the basis of computed and experimental frequencies the hydrogen-bridged structure 2 is most likely responsible for the experimentally observed frequencies in the solid state at −125 °C.

Comparative study of the superelectrophilic alkane dications (CnH2n+22+, n = 1–5) and their isoelectronic boron cation analogues was carried out using the ab initio method at the MP2/cc-pVTZ level. The structure, bonding, and relative stability of doubly charged alkane dications and monocharged boron cation analogues are discussed. These studies contribute to our general understanding of the superelectrophilic activation of alkyl cations as well as the electrophilic reactivity of C–H and C–C single bonds.

Good linear correlations between GIAO–CCSD(T) calculated 11B NMR chemical shifts of hypercoordinate boronium 1–6b and borenium 7–9b ions and 13C NMR chemical shifts of the corresponding isoelectronic carbonium 1–6a and carbenium 7–9a ions, respectively, were found.

C7H122+ (1), the prototype hexacoordinate carbonium dication was found to be a viable minimum at the MP2/6-31G** and MP2/cc-pVTZ levels. Structure 1 is a propeller shaped molecule resembling a complex involving a C2+ with three ethylene molecules resulting in the formation of three two-electron, three-center (2e–3c) bonds. Isomeric structure 2 was found to be 21.8 kcal/mol more stable than structure 1. However, conversion of 1 into 2 through transition structure 3 has a barrier of 5.7 kcal/mol. Related structures 4, 5, and 8 were also located as minima for C7H122+. The isoelectronic boron analogue BC6H12+ (10) was also computed to be a minimum at the same level of calculations.