Ab initio study of the adamantonium cations:: the protonated adamantane

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.