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Infrared spectra of SinH4n−1+ ions (n = 2–8): inorganic H–(Si–H)n−1 hydride wires of penta-coordinated Si in 3c–2e and charge-inverted hydrogen bonds
George, Martin Andreas Robert; Dopfer, Otto
SinHm+ cations are important constituents in silane plasmas and astrochemical environments. Protonated disilane (Si2H7+) was shown to have a symmetric three-centre two-electron (3c–2e) Si–H–Si bond that can also be considered as a strong ionic charge-inverted hydrogen bond with polarity Siδ+–Hδ−–Siδ+. Herein, we extend our previous work to larger SinH4n−1+ cations, formally resulting from adding SiH4 molecules to a SiH3+ core. Infrared spectra of size-selected SinH4n−1+ ions (n = 2–8) produced in a cold SiH4/H2/He plasma expansion are analysed in the SiH stretch range by complementary dispersion-corrected density functional theory calculations (B3LYP-D3/aug-cc-pVTZ) to reveal their bonding characteristics and cluster growth. The ions with n = 2–4 form a linear inorganic H–(Si–H)n hydride wire with adjacent Si–H–Si 3c–2e bridges, whose strength decreases with n, as evident from their characteristic and strongly IR active SiH stretch fundamentals in the range 1850–2100 cm−1. These 3c–2e bonds result from the lowest-energy valence orbitals, and their high stability arises from their delocalization along the whole hydride wire. For SinH4n−1+ with n ≥ 5, the added SiH4 ligands form weak van der Waals bonds to the Si4H19+ chain. Significantly, because the SinH4n−1+ hydride wires are based on penta-coordinated Si atoms leading to supersaturated hydrosilane ions, analogous wires cannot be formed by isovalent carbon.
