Structure and Stability of Diprotonated Methane, CH[formula omitted]

The prototype of hexacoordinate carbodications CH[formula omitted] has been investigated by ab initio molecular orbital theory with inclusion of electron-correlation effects. The geometries were determined with the HF/6-31G* basis set and their energies were calculated at the MP3/6-31G* level. A C_2v) symmetry structure with two stabilizing 3c-2e interactions is found to be a minimum on the potential energy surface. Two transition structures (C_2v) as well as several hilltop structures (D_3h, D_2d, C_5v, and O_h,) for polytopal intramolecular hydrogen rearrangements have been located. Hydrogen scrambling should be very rapid. CH[formula omitted] is thermodynamically unstable toward deprotonation, loss of H₂+·, and loss of H₃+ by 63.1, 18.6, and 126.8 kcal/mol, respectively. The deprotonation barrier was calculated to be 35.4 kcal/mol. A high barrier of 59.9 kcal/mol for loss of H₃+ was established. The heat of formation, δH^f(CH[formula omitted]), is estimated to be 651 ±3 kcal/mol. The kinetic stability combined with the favorable heat of hydrogenation of CH[formula omitted] of 79 kcal/mol suggests that CH[formula omitted] may be adaptable to experimental observation in the gas phase.