The Sulphur Dioxide-Water complex: CCSD(T)/CBS Anharmonic Vibrational Spectroscopy of Stacked and Hydrogen-Bonded Dimers

Abstract

<p>This study examines structures, energies, and IR vibrational spectra of the sulphur dioxide-water SO₂(H₂O) complexes by employing coupled cluster theory CCSD(T) with Dunning style correlation consistent type basis sets aug-cc-pV<em>n</em>Z (<em>n</em>=D,T,Q,5). Complete basis set (CBS) extrapolations have been carried out to predict binding energies for two isomers of the SO₂(H₂O) complex: a stacked global minimum (1A) and a hydrogen-bonded local minimum (1B) structure. CCSD(T)/CBS extrapolation predicts an intermolecular S-O distance <em>r</em>_S···O = 2.827 Å for the stacked isomer, which is in excellent agreement with an experimental measurement of 2.824 Å [K. Matsumura <em>et al.</em>, J. Chem. Phys., <strong>91</strong>, 5887, 1989]. The CCSD(T)/CBS binding energy for the stacked dimer 1A is D<em>_e</em>=4.43 kcal/mol and D<em>_e</em>=2.42 kcal/mol for the hydrogen-bonded form 1B. This study also employs anharmonic MP2/CBS VPT2 corrections to the CCSD(T)/CBS vibrational frequencies in both forms of SO₂(H₂O). The anharmonic CCSD(T)/CBS OH stretching frequencies in the stacked structure 1A are 3752 cm^-1 (n₃) and 3656 cm^-1 (n₁), and these align well with measured gas-phase SO₂(H₂O) values of 3745 cm^-1 and 3643 cm^-1, respectively [C. Wang <em>et al</em>., J. Phys. Chem. Lett., <strong>13</strong>, 5654, 2022]. If we combine CCSD(T)/aV<em>n</em>Z D<em>_e</em> with VPT2 vibrational frequencies, we obtain a new CCSD(T)/CBS anharmonic dissociation energy D₀=3.35 kcal/mol for 1A and D₀=1.67 kcal/mol in the case for 1B. In summary, the results presented here demonstrate that application of CCSD(T) calculations together with aV<em>n</em>Z basis sets and CBS extrapolations are critical in probing the structure and IR spectroscopic properties of the sulphur dioxide-water complex.</p>