Do H₅⁺ and Its Isotopologues Have Rotational Spectra?

High-level ab initio calculations predict a <i>C</i>_2<i>v</i> equilibrium geometry and large permanent dipole for H₅⁺, whereas rigorous Diffusion Monte Carlo calculations on a global potential surface show a completely symmetric zero-point averaged <i>D</i>_2<i>d</i> structure for H₅⁺, HD₄⁺, and D₅⁺, resulting in no permanent dipole moment. This dramatic departure from the conventional molecular structure is due to the highly fluxional nature of this cation. For the isotopologues H₄D⁺, H₃D₂⁺, and H₂D₃⁺, we predict nonzero dipole moments in the ground vibrational state and present corresponding simulated rotational spectra up to 3 THz. These predictions can guide the laboratory studies necessary for observational searches