The rotational spectrum of H 2 S⋯HI and an investigation by ab initio calculations of the origins of the observed doubling of rotational transitions in both H 2 S⋯HI and H 2 S⋯F 2 .

The rotational spectrum of the complex H ₂ S⋯HI observed with a pulsed-jet, Fourier-transform microwave spectrometer shows that each rotational transition is split into a closely spaced doublet, a pattern similar to that observed earlier for the halogen-bonded complex H ₂ S⋯F ₂ . The origin of the doubling has been investigated by means of ab initio calculations conducted at the CCSD(T)(F12 ^* )/cc-pVDZ-F12 level. Two paths were examined by calculating the corresponding energy as a function of two angles. One path involved inversion of the configuration at S through a planar transition state of C _2v symmetry via changes in the angle ϕ between the C ₂ axis of H ₂ S and the line joining the H and I nuclei [the potential energy function V(ϕ)]. The other was a torsional oscillation θ about the local C ₂ axis of H ₂ S that also exchanges the equivalent H nuclei [the potential energy function V(θ)]. The inversion path is slightly lower in energy and much shorter in arc length and is therefore the favored tunneling pathway. In addition, calculation of V(ϕ) for the series of hydrogen- and halogen-bonded complexes H ₂ S⋯HX (X = F, Cl, or Br) and H ₂ S⋯XY (XY = Cl ₂ , Br ₂ , ClF, BrCl, or ICl) at the same level of theory revealed that doubling is unlikely to be resolved in these, in agreement with experimental observations. The barrier heights of the V(ϕ) of all ten complexes examined were found to be almost directly proportional to the dissociation energies D _e.