INFRARED SPECTRA OF $CH_{3}^{+}$ -$Rg_{n}$), COMPLEXES ($Rg=He, Ne, Ar, n=1,2$)

$^{a}$ R. V. Olkhov, S. A. Nizkorodov and O. Dopfer J. Chem. Phys. 108, 10046, 1998.Author Institution: Institute for Physical Chemistry, University of BasleRotationally resolved infrared photodissociation spectra of $CH_{3}^{+}$-$Rg_{n}$ ionic complexers (Rg = He,Ne,Ar, n=1,2), have been recorded in a tendem mass spectrometer. The spectra of the dimers are compatible with $\pi$--bonded equilibrium structures where the Rg atoms are attached to the vacant $2p_{z}$ orbital of the central C atom ($C_{3v}$ symmetry). In the case of Ar, partial charge transfer into this orbital leads to a large binding energy of the complex (ca. 0.5 eV), so that only overtones of the CH stretch modes can be observed in the photofragmentation spectra. The strong intermolecular bond induces a massive deformation of the $CH_{3}+$ ion which transforms from $sp^{2}$ towards $sp^{3}$ hybridization. In the $Ar-CH_{3}+$-Ar trimer the second Ar atom is weakly attached to the other side of the $2p^{z}$ orbital of C ($C_{3v}$ symmetry). The two C-Ar bonds are not equivalent: one is strong and short, the other one is long and weak. However, tunneling splittings reveal that they can exchange their role via the intracluster inversion motion of $CH_{3}^{+}$ through a transition state with $D_{3h}$ $symmetry^{a}$. The intermolecular bonds in the He and Ne containing complexes are much weaker and result mainly from induction interactions. The $CH_{3}^{+}$ deformation is smaller and the absence of the tunneling splittings in the $Ne-CH_{3}^{+}$-Ne spectrum suggests that the trimers have $D_{3h}$ equilibrium geometries. The experimental results are supported by ab initio calculations