Do H5+ and Its Isotopologues Have Rotational Spectra?

High-level ab initio calculations predict a C-2v equilibrium geometry and large permanent dipole for H-5(+), whereas rigorous Diffusion Monte Carlo calculations on a global potential surface show a completely symmetric zero-point averaged D-2d structure for H-5(+), HD4+, and D-5(+), 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.