Ab initio study of the adamantonium cations: the protonated adamantane

The molecular structure and energetics of the adamantonium ions were comput ed at the MP2(full)/6-31G** level. Three structures were found to represent the adamantonium cations, respectively: the 1-H-adamantonium (1). 2-H-adam antonium (2), and C-adamantonium ions (3). This study revealed that, upon p rotonation, adamantane can also produce two van der Waals complexes: one fo rmed 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 co mplexity of this molecule, the quantum zero point energy (ZPE) and finite t emperature (298 K) corrections were estimated from previously calculated va lues for the isobutonium (for protonation of tertiary C-H and C-C bonds) an d the proponium cations (for protonation of the secondary C-H bond). The ca lculated proton affinity of adamantane was estimated as 175.7 kcal/mol.