Deciphering Noncovalent Interactions in Preassociation Complexes of Water, CO₂, DMSO with N‑Methylmorpholine N‑Oxide:...

Preassociation complex formation is a key initial step in the sequestration of CO₂ in the presence of water and DMSO. In the present study, we first analyzed the conformers of N-methylmorpholine N-oxide (MMNO), as they can affect the preassociation complex formation. The stability order of the CO₂, H₂O, and DMSO adsorbed complexes across various conformers was identified. Our findings indicate that the adsorbates bind to the exocyclic oxygen atom of the MMNO molecule, while the oxygen atom of the adsorbate simultaneously interacts with the methyl and methylene hydrogen atoms of MMNO. Binding is more favorable in the nonchair conformers of MMNO due to its flexibility. The binding energies calculated at the DLPNO-CCSD-(T) level are similar for H₂O and DMSO, whereas CO₂ exhibits a value approximately half that of H₂O and DMSO. The formation of CO₂ complexes with all conformers is associated with a positive free-energy change, indicating that the process is not thermodynamically favorable. The computed enthalpy values indicate that their formation is enthalpy-driven. The solvation energies are highest for water, lower for DMSO, and lowest for CO₂. The CO₂ complex shows negative H(r) and positive ∇²ρ-(r), indicating that the potential energy dominates kinetic repulsion and can be characterized as partially covalent with some degree of electron sharing, with the existence of a strong N-O···C tetrel bond. In the CO₂ complexes, the tetrel bond is the strongest, and the hydrogen bond is the weakest. In the CO₂ complexes, a red region surrounding the blue region was observed, indicative of steric repulsion that contributes to their destabilization. Ab initio molecular dynamics (AIMD) analysis of the CO₂ complex in the presence of 30 explicit DMSO molecules shows that CO₂ molecules tumble, are localized near the first solvation shell, and have a low probability of escaping it. Overall, the work highlights the binding modes and energetics of adsorbates such as CO₂, H₂O, and DMSO with various MMNO conformers, as well as the nature of their interactions.