Av ery broad acidity scale ( % 40 pK units) for about 400 N-heterocyclic carbenep recursors (NHCPs) with various backbones and electronic features, including imidazolylidenes, 1,2,4-triazolylidenes, cyclic diaminocarbenes (CDACs), diamidocarbenes (DACs), thiazolylidenes, cyclic (alkyl)(amino)carbenes (CAACs) and mesoionic carbenes (MICs), was established in DMSO by aw ell examined computational method. Varying the backbone structure or flanking N-substituents can have different extent of acidifying effects, de-pending on both the nature and number of substituent(s). The Gibbs energies (DG r s) for the reactions between the corresponding NHCs and CO 2 were also calculated.T here is a good linear correlation between the pK a so fm ostN HCPs and DG r s, suggestingt hat ag reater basicity of NHC leads to am ore stable NHC-CO 2 adduct. Interestingly,t he nearby asymmetrice nvironment has virtually no differential effect on the acidities of the chiral NHCP enantiomers, but has a pronounced effect on the DG r values.[a] Dr.