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

Abstract

Si_nH_m⁺ 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 Si^δ+–H^δ−–Si^δ+. Herein, we extend our previous work to larger Si_nH_4n−1⁺ cations, formally resulting from adding SiH₄ molecules to a SiH₃⁺ core. Infrared spectra of size-selected Si_nH_4n−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 Si_nH_4n−1⁺ with n ≥ 5, the added SiH₄ ligands form weak van der Waals bonds to the Si₄H₁₉⁺ chain. Significantly, because the Si_nH_4n−1⁺ hydride wires are based on penta-coordinated Si atoms leading to supersaturated hydrosilane ions, analogous wires cannot be formed by isovalent carbon.