Intermolecular interaction in the CH 3 1 – He ionic complex revealed by ab initio calculations and infrared photodissociation spectroscopy

The infrared photodissociation spectrum of the degenerate asymmetric CH stretch (n3) vibration of the CH3 –He ionic complex has been recorded. The rotational structure and vibrational frequency of the observed transition are consistent with a p-bonded C3v cluster geometry where the He ligand is attached to the 2pz orbital of the central C atom of CH3 1 . The intermolecular bond in the ground vibrational state is characterized by an averaged intermolecular separation of Rcm52.18 Å. The origin of the n3 vibration of the complex is slightly blue shifted ~7 cm ! 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 p-bonded configuration corresponds to the global minimum structure of the complex ~De 5707 cm, Re51.834 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, Rcm2Re50.35 Å. © 1999 American Institute of Physics. @S0021-9606~99!02319-3#