Intermolecular interaction in the CH3+-He ionic complex revealed by ab initio calculations and infrared photodissociation spectroscopy

The infrared photodissociation spectrum of the degenerate asymmetric CH str etch (nu(3)) vibration of the CH3+-He ionic complex has been recorded. The rotational structure and vibrational frequency of the observed transition a re consistent with a pi-bonded C-3v cluster geometry where the He ligand is attached to the 2p(z) orbital of the central C atom of CH3+. The intermole cular bond in the ground vibrational state is characterized by an averaged intermolecular separation of R-cm=2.18 Angstrom. The origin of the nu(3) vi bration of the complex is slightly blue shifted (7 cm(-1)) compared to the monomer frequency, indicating that vibrational excitation is accompanied by a slight destabilization of the intermolecular bond. Ab initio calculation s at the MP2/aug-cc-pVTZ(#) level of theory confirm that the pi-bonded conf iguration corresponds to the global minimum structure of the complex (D-e=7 07 cm(-1), R-e=1.834 Angstrom). The calculated intermolecular potential ene rgy surface of this "disk-and-ball" ionic complex reveals substantial angul ar-radial couplings in the region of the global minimum, which account for the large discrepancy between vibrationally averaged and calculated equilib rium intermolecular separations, R-cm-R-e=0.35 Angstrom. (C) 1999 American Institute of Physics. [S0021-9606(99)02319-3].