IR photodissociation spectra of SixH4x-4+ (x?=?4–8): Evidence for Si H Si proton bridges

摘要： Although silane-type cations of the form SixHy+ are thought to play a significant role in plasma chemistry and astrochemistry, their structural and energetic properties are largely unexplored in the size range x ≥ 3. Herein, infrared photodissociation (IRPD) spectra of mass-selected SixHy+ with x = 4–8, specifically SixH4x-4+, are recorded in the Si–H stretch range and analyzed by dispersion-corrected density functional calculations at the B3LYP-D3/aug-cc-pVDZ level. The SixH4x-4+ ions are produced in a SiH4/H2/He plasma molecular beam expansion. The IRPD process leads to the loss of SiH4 ligands, which corresponds to the lowest-energy fragment channel. Spectral analysis of the IRPD spectra reveals that all SixH4x-4+ ions have at least one Si–H–Si bridge. The characteristic fingerprint of these three-center two-electron (3c-2e) bonds is the highly IR active antisymmetric stretch fundamental of the Si–H–Si bridge (νSiHSi) occurring in the 1600–2100 cm?1 range, whose frequency strongly depends on the structural and energetic details of the Si–H–Si bridge. Although the investigated SixH4x-4+ ions can formally be described by the formula Si2H4+(SiH4)x-2, the cluster growth is more complex. The appearance of the νSiHSi bands confirms that all considered SixH4x-4+ ions with x ≤ 5 are formed by polymerization reactions. Larger clusters (x ≥ 6) show evidence for the additional presence of weakly-bonded SiH4 ligands attached to smaller chemically-bonded core ions. Correlations of the properties of the Si–H–Si bridges (bond distances, bond angles, binding energies, Si–H stretch frequencies), which vary between strong symmetric 3c-2e chemical bonds and weak hydrogen bonds, are discussed.