Infrared photodissociation spectroscopy of [Si.sup.+][(C[O.sub.2]).sub.n] and [Si.sup.+][(C[O.sub.2]).sub.n]Ar complexes--evidence for unanticipated intracluster reactions

[Si.sup.+][(C[O.sub.2]).sub.n] and [Si.sup.+][(C[O.sub.2]).sup.n] Ar ion-molecule complexes were produced by laser vaporization in a pulsed supersonic expansion. The ions were mass-selected in a reflection time-of-flight spectrometer and studied with infrared photodissociation spectroscopy near the asymmetric stretch vibration of C[O.sub.2]. [Si.sup.+][(C[O.sub.2]).sub.n] clusters fragment by the loss of C[O.sub.2] whereas [Si.sup.+][(C[O.sub.2]).sub.n]Ar complexes fragment by the loss of argon. All clusters have resonances near the C[O.sub.2] asymmetric stretch, but with shifts in frequency that are size dependent. The patterns seen in the small clusters are consistent with electrostatic bonding, while the larger systems provide evidence for an intracluster reaction forming oxide-carbonyl species. Density functional theory was employed to examine the structures of these clusters, and their calculated vibrational frequencies were compared to the measured values. Ligand assembly in [Si.sup.+][(C[O.sub.2]).sub.n] complexes is dominated by the presence of the occupied 3p valence orbital of the silicon cation. Key words: ion-molecule complexes, infrared spectroscopy, photodissociation, density functional theory.