Acidity Scale of N‑Heterocyclic Carbene Precursors: Can We Predict the Stability of NHC–CO2 Adducts?

The acidity scale (∼14 pK units) of more than 90 triazolium, imidazolium, and imidazolinium based NHC precursors in DMSO was established systematically by a well-developed computational model. The substituent effects on the acidities of these NHC precursors were analyzed through acidity comparison and Hammett correlation. The binding energy (ΔG 1) of the reaction between NHC and CO2 was also calculated and linearly correlates with the basicity of the corresponding NHC, which implies that the stability of the NHC–CO2 adduct is essentially dictated by the basicity of NHC.