Structures and Energetics of E₂H₃⁺ (E = As, Sb, and Bi) Cations

E₂H₂ (E = As, Sb, Bi) structures involving multiple bonds have attracted much attention recently. The E₂H₃⁺ cations (protonated E₂H₂) are predicted to be viable with substantial proton affinities (>180 kcal/mol). Herein, the bonding characters and energetics of a number of E₂H₃⁺ isomers are explored through CCSD(T) and DFT methods. For the As₂H₃⁺ system, the CCSD(T)/cc-pVQZ-PP method predicts that the vinylidene-like structure lies lowest in energy, with the trans and cis isomers higher by 6.7 and 9.3 kcal/mol, respectively. However, for Sb₂H₃⁺ and Bi₂H₃⁺ systems, the trans isomer is the global minimum, while the energies of the cis and vinylidene-like structures are higher, respectively, by 2.0 and 2.4 kcal/mol for Sb₂H₃⁺ and 1.6 and 15.0 kcal/mol for Bi₂H₃⁺. Thus, the vinyledene-like structure is the lowest energy for the arsenic system but only a transition state of the bismuth system. With permanent dipole moments, all minima may be observable in microwave experiments. Besides, we have also obtained transition states and planar-cis structures with higher energies. The current results should provide new insights into the various isomers and provide a number of predictions for future experiments.