The structure and energetics of the n-butonium ion, the protonated form of n-butane, were computed at the MP4SDTQ(fc)/6-311++G**//MP2(full)/6-31G** level. Eleven stable structures were found for the n-butonium ion, following the stability order 2-C-n-butonium > 1-C-n-butonium > 2-H-n-butonium > 1-H-n-butonium. The transition states for intramolecular bond-to-bond rearrangement and for decomposition of the carbonium ions into the van der Waals complexes were also calculated. The H-n-butonium and the 1-C-n-butonium ions are higher in energy than the van der Waals complexes 13, 14, and 15. The van der Waals complexes between the isopropyl cation plus CH 4 and the tert-butyl cation plus H 2 are the most stable C 4 H 11 + species. It was concluded that the 1-H-n-butonium ion prefers to undergo intramolecular rearrangement to the 1-C-n-butonium ion, whereas the 2-H-n-butonium ion prefers to decompose into the van der Waals complex of the sec-butyl cation plus H 2 . The calculated proton affinity of n-butane (156.7 kcal/mol) agrees well with the experimental value of 153.7 kcal/mol. The C 4 H 11 + (b) species, formed upon the gas-phase reaction between C 2 H 5 + and ethane, was confirmed to be the 2-C-n-butonium cation, and the C 4 H 11 + (a) species was confirmed to be the 2-H-n-butonium cation, as proposed by Hiraoka and Kebarle (Can. J. Chem. 1980Chem. , 58, 2262Chem. -2270. The experimental activation energy of 9.6 kcal/mol was compared with the value of 12.8 kcal/mol, computed for the reaction 11 f 5 through the transition state 21.

The energy of the n-butonium and isobutonium cations was calculated. At the MP4/6-311++G**//MP2(fu)/6-31G** level, the C-carbonium ions were more stable than the H-carbonium ions. The results are in agreement with gas-phase data of n-butane and isobutane protonation but disagree with results in liquid superacid, where protonation of the tertiary C−H of isobutane is preferred over C−C protonation. Additional calculations, including the superacid moiety, revealed that the activation energy for C−C protonation is higher than the energy for attack at the tertiary C−H. This suggests that the σ bond reactivity in the liquid superacid system is controlled by the activation energy for proton transfer, rather than by the intrinsic basisity of the bond. The higher stability of the C-carbonium relative to the H-carbonium ions was ascribed to a better charge distribution among the atoms and groups of the three center bond.

The molecular structure and energetics of the adamantonium ions were computed at the MP2(full)/6-31G** level. Three structures were found to represent the adamantonium cations, respectively: the 1-H-adamantonium (1), 2-H-adamantonium (2), and C-adamantonium ions (3). This study revealed that, upon protonation, adamantane can also produce two van der Waals complexes: one formed by the weak interaction of the 1-adamantyl cation and H 2 (4) and the other formed by the interaction of the 2-adamantyl cation and H 2 (5). The stability order is predicted to be 5 > 3 > 4 > 1 > 2. Given the size and complexity of this molecule, the quantum zero point energy (ZPE) and finite temperature (298 K) corrections were estimated from previously calculated values for the isobutonium (for protonation of tertiary C-H and C-C bonds) and the proponium cations (for protonation of the secondary C-H bond). The calculated proton affinity of adamantane was estimated as 175.7 kcal/mol.