The rotational spectrum of H2S⋯HI and an investigation by ab initio calculations of the origins of the observed doubling of rotational transitions in both H2S⋯HI and H2S⋯F2

Andrew P Suckley; David P Tew; Anthony C Legon

doi:10.1063/5.0028222

The rotational spectrum of H₂S⋯HI and an investigation by ab initio calculations of the origins of the observed doubling of rotational transitions in both H₂S⋯HI and H₂S⋯F₂

J Chem Phys. 2020 Nov 28;153(20):204301. doi: 10.1063/5.0028222.

Authors

Andrew P Suckley¹, David P Tew², Anthony C Legon³

Affiliations

¹ Department of Chemistry, University of Exeter, Stocker Road, Exeter EX4 4QD, United Kingdom.
² Physical and Theoretical Chemistry Laboratory, University of Oxford, South Parks Road, Oxford OX1 3QZ, United Kingdom.
³ School of Chemistry, University of Bristol, Cantock's Close, Bristol BS8 1TS, United Kingdom.

PMID: 33261470
DOI: 10.1063/5.0028222

Abstract

The rotational spectrum of the complex H₂S⋯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 H₂S⋯F₂. 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 C₂ axis of H₂S 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 C₂ axis of H₂S 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 H₂S⋯HX (X = F, Cl, or Br) and H₂S⋯XY (XY = Cl₂, Br₂, 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.