Infrared spectroscopy of Si(CO)n+ complexes: evidence for asymmetric coordination.

Si(CO)(n)(+) and Si(CO)(n)(+)Ar complexes are produced via laser vaporization with a pulsed nozzle source and cooled in a supersonic beam. The ions are mass selected in a reflectron time-of-flight mass spectrometer and studied with infrared laser photodissociation spectroscopy near the free molecular CO vibration (2143 cm(-1)). Si(CO)(n)(+) complexes larger than n = 2 fragment by the loss of CO, whereas Si(CO)(n)(+)Ar complexes fragment by the loss of argon. All clusters have resonances near the free molecular CO stretch that provide distinctive patterns from which information on their structure and bonding can be obtained. The number of infrared-active bands, their frequency positions, and relative intensities indicate that larger species consist of an asymmetrically coordinated Si(CO)(2)(+) core with additional CO ligands attached via van der Waals interactions. Density functional theory computations are carried out in support of the experimental spectra.