Quantum Chemical Studies On The Interaction Of Oxygen And Water Clusters

The dioxygen which is triplet state belongs to open-shell system. It is a big challenging subject to calculate the intermolecular interaction of open-shell systems exactly based on theory. What's more, there is a growing attention for the dioxygen clathrate hydrates because of their important applications in biomolecular function, materials synthesis and gas mixture separation.The equilibrium geometries, interaction energies and the total interaction energies of the interaction between cage-like complexes [?H₂O?n, n=8,10,12,14,16,18,20,24] and singlet?1?g?,triplet dioxygen were calculated with the nonlocal correlation and range-separated hybrid density functional ?B97X-V which is very successful in the calculation of intermolecular interaction. The many-body interaction energies of ^TO₂@?H₂O?₂₀ and ^SO₂@?H₂O?₂₀ were calculated as well, and the many-body effects were analyzed based on that. The Gibbs free energies, enthalpies, entropies of ^TO₂@?H₂O?₂₀ and ^SO₂@?H₂O?₂₀ was calculated at M06-2X/6-311++G?3df,3pd?. Furthermore, the energy decomposition analysis of interaction energies for [^SO₂@?H₂O?n, n=16,18,20,24] was calculated at SAPT0/aug-cc-p VTZ, which revealed the nature of their interaction.The calculated results showed that ^TO₂@?H₂O?₂₀ was the most stable one in the triplet dioxygen clathrate hydrates, however, the stability of ^SO₂@?H₂O?₂₀ was as same as ^SO₂@?H₂O?18, and they both were the most stable ones in the singlet dioxygen clathrate hydrates according to the mean binding energies derived from the total interaction energies.The two-body interaction energy contributed 73.0% to the total interaction energy, the three-body interaction energy contributed 19.7%, and the sum of four-, many-body interaction energies accounted for only 7.3% for ^TO₂@?H₂O?₂₀. The two-body interaction energies contributed up to 75.0%, the proportion of three-body interaction energies was 8.2%,and the percentage of four-, many-body interaction energies increased to 16.8% for ^SO₂@?H₂O?₂₀.According to the gas phase Gibbs free energies calculated with the density functional theory M06-2X, it was impossible to form spontaneously for ^TO₂@?H₂O?₂₀ and ^SO₂@?H₂O?₂₀at 1 atm, 298.15 K. The critical temperature of ^TO₂@?H₂O?₂₀ forming spontaneously was266.43 K; while ^SO₂@?H₂O?₂₀ worked at 265.72 K.The SAPT0 analyses clearly showed that the percentage of dispersion energy in the total attractive interactions of [^SO₂@?H₂O?n, n=16, 18, 20, 24] was 76.2%, 77.6%, 84.9%, 88.9%,respectively, which was 4¹0 times bigger than that of electrostatic portion. The proportion of electrostatic portion was 19.6%, 18.5%, 12.4%, 8.8%, respectively. The induction contribution to the interaction energy was 4.1%, 3.9%, 2.7%, 2.3%, respectively. Dispersion type interaction played the dominant role for their stability, the contribution of electrostatic portion was smaller that of dispersion, and the induction contribution was the smallest. The hydrogen bonds between singlet dioxygen and cage-like complexes were very weak, and the induction interaction between dioxygen and water molecules was not very remarkable.