In light of the recent discovery of ArH⁺ in the Crab nebula, it is shown through high-level quantum chemical comptuations that the global minima on the Ar⁺/Ne⁺ + H₂ potential energy surfaces are ArH₂⁺ and NeH₂⁺. Hence, ArH₂⁺ may be a necessary intermediate in the Ar⁺ + H₂ → ArH⁺ + H formation reaction proposed in the same work where ArH⁺ is first reported in the Crab nebula. ArH₂⁺ is also probably an intermediate in the alternative $$\mathrm{Ar} + \mathrm{H}_2^+ \rightarrow \mathrm{ArH}^+ + \mathrm{H}$$ reaction. Additionally, it is shown that Ne$$^+ + \mathrm{H}_2 \rightarrow \mathrm{NeH}_2^+$$ will subsequently most likely yield $$\mathrm{Ne} + \mathrm{H}_2^+$$ and not NeH⁺ + H offering a possible rationale as to the absence of NeH⁺ in spectra obtained from the interstellar medium (ISM). Following from this, spectroscopic data (both rotational and vibrational) are provided for NeH₂⁺ and ArH₂⁺ through the use of highly accurate quantum chemical quartic and cubic force fields. All possible isotopologues are also included for ²⁰Ne, ²²Ne, ³⁶Ar, ³⁸Ar, ⁴⁰Ar, ¹H, and D. The dipole moments for these systems are quite large at 5.61 D for NeH₂⁺ and 4.37 D for ArH₂⁺. The spectroscopic constants provided will aid in the potential detection of these open-shell noble gas dihydride cations in the ISM.

Sulfurous acid (H₂SO₃) is an infamously elusive molecule. Although some theoretical papers have supposed possible roles for it in more complicated systems, it has yet to be experimentally observed. To aid experiment in detecting this molecule, we have examined the H₂O + SO₂ potential energy surface at the CCSDT(Q)/CBS//CCSD(T)-F12b/cc-pVTZ-F12b level of theory to resolve standing discrepancies in previous reports and predict the gas-phase vibrational spectrum for H₂SO₃. We find that sulfurous acid has two potentially detectable rotamers, separated by 1.1 kcal mol⁻¹ ΔH_0K with a torsional barrier of 1.6 kcal mol⁻¹. The sulfonic acid isomer is only 6.9 kcal mol⁻¹ above the lowest enthalpy sulfurous acid rotamer, but the barrier to form it is 57.2 kcal mol⁻¹. Error in previous reports can be attributed to misidentified stationary points, the use of density functionals that perform poorly for this system, and, most importantly, the basis set sensitivity of sulfur. Using VPT2+K, we determine that the intense S=O stretch fundamental of each species is separated from other intense peaks by at least 25 cm⁻¹, providing a target for identification by infrared spectroscopy.