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

Abstract

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+ ions (specifically SixH4x-4+ with x = 4–8, Si4H12+-Si8H28+) are recorded in the SiH 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 SiHSi bridge. The characteristic fingerprint of these three-center two-electron (3c-2e) bonds is the highly IR active antisymmetric stretch fundamental of the SiHSi bridge (σ SiHSi) occurring in the 1600–2100 cm−1 range, whose frequency strongly depends on the structural and energetic details of the SiHSi 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 σSiHSi 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 SiHSi bridges (bond distances, bond angles, binding energies, SiH stretch frequencies), which vary between strong symmetric 3c-2e chemical bonds and weak hydrogen bonds, are discussed.