The ozone-water complex: CCSD(T)/CBS structures and anharmonic vibrational spectroscopy of O3(H2O)n, (n = 1 - 2)
Wallace C H Hui1, Kono H Lemke1
1Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Pok Fu Lam, Hong Kong, SAR.
Abstract:
Ozone-water complexes O3(H2O)n (n = 1-2) have been studied using coupled cluster theory with triple excitations CCSD(T) with correlation consistent basis sets aug-cc-pVnZ (n = D, T, Q) and complete basis set (CBS) extrapolation techniques. We identified seven dimer (n = 1) and nine trimer species (n = 2) with open C2v and cyclic D3h ozone. Calculations at the CCSD(T)/CBS level of theory for C2v O3(H2O) on the counterpoise (CP)-corrected potential energy surface yield a dissociation energy of De = 2.31 kcal/mol and an O3 central-oxygen (Oc) H2O oxygen (Ow) distance r[Oc⋯Ow] of 3.097 Å, which is in good agreement with an experimental value of 2.957 Å [J. Z. Gillies et al., J. Mol. Spectrosc. 146, 493 (1991)]. Combining our CCSD(T)/CBS value of De for C2v O3(H2O) with our best estimate anharmonic CCSD(T)/aVTZ ΔZPE yields a Do value of 1.82 kcal/mol; the CCSD(T)/CBS value of De for D3h O3(H2O) is 1.51 kcal/mol and yields an anharmonic CCSD(T)/aVTZ Do = 0.99 kcal/mol. CCSD(T)/aVTZ dissociation energies and structures for C2v O3(H2O)2 are De = 4.15 kcal/mol, (Do = 3.08 kcal/mol) and r[Oc⋯Ow] = 2.973 Å, and De = 2.64 kcal/mol (Do = 1.68 kcal/mol) with r[Oc⋯Ow] = 2.828 Å for D3h O3(H2O)2. The results from ab initio molecular dynamics simulations, which consider dynamic and thermal effects in O3(H2O), show that the O3(H2O) complex remains stable at 50 K and dynamically interconverts between two hydrogen-bonded conformers with short Oc⋯Ow contacts (3.85 Å). Carr-Parrinello molecular dynamic (CPMD) simulations for O3(H2O) and O3(H2O)2 at 100 K demonstrate that O3(H2O)2 remains structurally intact, whereas O3(H2O) dissociates to free ozone and water, a feature consistent with the larger average binding energy in O3(H2O)2 (2.2 kcal/mol) vs that in O3(H2O) (1.8 kcal/mol). Finally, the results from CCSD(T)/CBS and CPMD simulations demonstrate that the large inter-trimer binding energies in O3(H2O)2 would give rise to an elevated trimer/dimer population ratio, making O3(H2O)2 a particularly stable and spectroscopically detectable complex.
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