NO₂ Solvation Structure in Choline Chloride Deep Eutectic Solvents-The Role of the Hydrogen Bond Donor

Deep eutectic solvents (DESs) are promising candidates as alternate media for industrial gas sequestration processes, such as denitrification via NO₂ adsorption. Here, quantum chemical methods are employed to characterize the NO₂ solvation structure and adsorption mechanism in choline chloride DES with urea, methylurea, and thiourea hydrogen bond donors. Our results show that the solvation structure of NO₂ in bulk choline chloride DES is determined by the type of the hydrogen bond donor present. Changing the structure of the hydrogen bond donor not only changes its NO₂ coordination mechanism but also changes that between NO₂ and the choline and chloride ions in the DES. By using an energy decomposition analysis scheme, we show that the principle forces stabilizing NO₂ in these DES are dispersion and polarization interactions and, consequently, that NO₂ adsorption is most favorable in the choline chloride-thiourea DES. These results highlight a potential route for the optimization of choline chloride DES for denitrification, by modulating the hydrogen bond donor structure.