Microsolvation of the methyl cation in neon: Infrared spectra and ab initio calculations of CH3+-Ne and CH3+-Ne-2

Rotationally resolved infrared photodissociation spectra of the degenerate asymmetric C-H stretch vibration (nu(3)) of the CH3+-Ne and CH3+-Ne-2 ionic complexes have been recorded. The rotational structure and vibrational fre quencies are consistent with pi-bound cluster geometries, where the Ne liga nds are attached to either side of the 2p(z) orbital of the central C atom of the methyl cation, leading to C-3v and D-3h symmetric structures for the dimer and trimer. The intermolecular bonds in the ground vibrational state are characterized by averaged separations of R-c.m.=2.30 Angstrom in the d imer and 2.34 Angstrom in the trimer. The origins of the nu(3) band are blu eshifted by 11.5 and 21.5 cm(-1) compared to the monomer frequency, indicat ing that vibrational excitation is accompanied by a small and additive dest abilization of the intermolecular bond. Ab initio calculations at the MP2/a ug-cc-pVTZ(#) level confirm that the pi-bound configurations correspond to the global minimum structures for both the dimer (D-e=958.5 cm(-1), R-e=2.1 347 Angstrom, theta(e)=91.4 degrees) and the trimer (D-e=745.4 cm(-1), R-e= 2.2322 Angstrom, theta(e)=90 degrees). The calculated intermolecular potent ial energy surface of the dimer is characteristic for a disk-and-ball compl ex and reveals significant angular-radial coupling, which accounts for the large discrepancy between the vibrationally averaged and calculated equilib rium intermolecular separations, R-c.m.-R(e)approximate to 0.17 Angstrom. T he comparison of the ionic CH3+-Rg dimers (Rg=He, Ne, Ar, Kr, Xe) with the isoelectronic CH3X molecules (X=H, F, Cl, Br, I) reveals that chemical bond ing onsets with Rg=Ar and increases with the size of the Rg atom. (C) 2000 American Institute of Physics. [S0021-9606(00)00305-6].