Structure, vibrational spectrum, and energetics of the CH+5 ion. A theoretical investigation

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Abstract
A set of large-scale ab initio molecular orbital calculations on the CH+5 potential energy surface is presented. The final calculations were done with very large polarized basis sets and correlation corrections were included at the CI(SDQ) level. The geometries were gradient optimized, while the harmonic force fields and infrared intensities were also determined analytically at the SCF level. None of the modes are predicted to have an extremely large intensity. The frequency of the torsional mode for rotation of the H2 about the pseudo threefold axis of the CH+3 group is found to be extremely low. This makes the exact energy content of the ion difficult to define precisely. The proton affinity of CH4 has been calculated at the CI(SDQ) level including zero point energy and thermodynamic corrections. The calculated value is 129.0 kcal/mol as compared to an experimental value of 132.4 kcal/mol. It is suggested that the experimental value is somewhat too high.