Sulfurous and sulfonic acids: Predicting the infrared spectrum and setting the surface straight

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.