Infrared spectrum of the silicon hydride cation SiH₇^PLU

Silanium ions are an important class of hypervalent molecules, and the determination of their structure will yield insights into the nature of nonclassical bonding and provide a contrast to the bonding in carbonium ions. We report the infrared spectrum of the mass-selected silicon hydride cluster ion ²⁸SiH₇⁺, detected by vibrational predissociation spectroscopy. Silanium ions were formed in a pulsed high pressure glow discharge and cooled by the subsequent supersonic expansion. Photodissociation spectra were obtained using a tandem time-of-flight mass spectrometer: SiH₇⁺ ions were mass-selected and excited by a tunable infrared laser. The resulting photofragments were detected using a reflectron as a mass analyzer. We observed a vibrational band at 3865 cm^-1, which was the only one observed from 3500 cm^-1 to 4200 cm^-1. This result suggests that the molecule might form a symmetric complex with the structure H ₂·SiH₃⁺·H₂, in contrast to the species CH ₇⁺, which has the structure CH₅⁺·H₂.