The Unique Vinylidene-Like Global Minimum of the Si 2 H 2 -

This study presents a high-level computational investigation into the structural, vibrational, and energetic properties of neutral and anionic Si 2 H 2 structures. Through rigorous focal-point analysis at the CCSD(T) level and beyond, we demonstrate that electron attachment fundamentally reshapes the potential energy surface with computations using basis sets as large as quadruple zeta, and correlation treatments as sophisticated as CCSDT(Q). While the neutral butterfly is lowest in energy and has been identified experimentally, the anionic surface is fundamentally different. Our results remarkably identify the vinylidene structure as the lowest-energy anionic minimum, significantly preferred over the butterfly structure, which fails to support an anion minimum. The anion supports two minima: the vinylidene-like and monobridged; the vinylidene structure is lower in energy by 12.2 kcal/mol. The electron adiabatic affinity, the energy difference between the neutral butterfly and the anion vinylidene-like structure is 27.6 kcal/mol. This structural rearrangement upon electron attachment is related to behavior observed in isoelectronic hydrocarbons and other silicon hydrides, where charge accommodation forces a collapse to a more favorable semi-bridged ground-state minimum. By characterizing the distinct geometric, vibrational, and energetic signatures of these isomers, this research provides a definitive basis for the experimental detection of the anionic Si 2 H 2 species and adds a new chapter to the unique bonding behavior that distinguishes silicon from its lighter group 14 counterparts.