Infrared spectrum of the silicon hydride cation SiH_7^+

Abstract

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 ^(28)SiH_7^+ 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: (formula available in paper) 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⋅SiH_3^+⋅H_2, in contrast to the CH_7^+ which has the structure CH_5^+⋅H_2.

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