New insights into the chemistry of ionic alkylorganic carbonates: a computational study

A library of hydrogenated, perfluorinated aliphatic and aromatic (p-substituted) alcohols are selected together with a combination of superbases (SBs) and metal hydrides (MHs) to understand the thermodynamic parameters of the binary mixtures once serving as sorbents for the capture of CO₂via ionic organic alkyl-carbonate (RCO₃^-) formation. Data are obtained using density functional theory (DFT) calculations with the B3LYP/6-31+G* level of theory and compared with the experimental results acquired from the literature using different spectroscopic techniques. It is found that the capturing process has a favourable enthalpic contribution and an unfavourable entropic penalty regardless the identity of the base, where the enthalpy values of alcohol/MH binary mixtures are almost two-fold higher compared to their SB-based mixtures. The utilisation of perfluorinated aliphatic alcohols instead of hydrogenated alcohols shows a negative impact on the formation of carbonate adducts, due to the less reactive alkoxide anion along the carbon skeleton, which is attributed to the low charge density of the nucleophilic oxygen atom. While perfluorinated phenol shows a higher reactivity than the parent phenol. The calculations indicate that the reactivity of phenolic compounds is highly affected by the electronic nature of the substituting groups, in which p-substituted phenols are more reactive towards CO₂ capturing when electron releasing groups are utilised. A pronounced solvent effect is observed, in which the alkylcarbonate salts (RCO₃^- SBH⁺) are stabilized in solvents with high dielectric constant (e.g., DMSO and MeCN). Simulated NMR and IR spectra of RCO₃^- are consistent with those reported for the affiliated systems, which fortifies the results obtained for the unexplored substrate/MH mixtures, filling a gap in the literature of CO₂ sequestration using CO₂ binding organic liquids (CO₂BOLs) and enabling a fair/quick prediction of potential substrates to be used as CO₂ sorbents.