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 stretch (ν3) vibration of the CH3+–He ionic complex has been recorded. The rotational structure and vibrational frequency of the observed transition are consistent with a π-bonded C3v cluster geometry where the He ligand is attached to the 2pz orbital of the central C atom of CH3+. The intermolecular bond in the ground vibrational state is characterized by an averaged intermolecular separation of Rcm=2.18 Å. The origin of the ν3 vibration 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 calculations at the MP2/aug-cc-pVTZ# level of theory confirm that the π-bonded configuration corresponds to the global minimum structure of the complex (De=707 cm−1, Re=1.834 Å). The calculated intermolecular potential energy surface of this “disk-and-ball” ionic complex reveals substantial angular-radial couplings in the region of the global minimum, which account for the large discrepancy between vibrationally averaged and calculated equilibrium intermolecular separations, Rcm−Re=0.35 Å.