Computational Estimation of the Gas-Phase and Aqueous Acidities of Carbon Acids

Carbon acids are compounds which ionize by dissociating along carbon-hydrogen bonds. Although commonly noted for the extremely low acidities of some members of this class, these compounds in fact display a wide range of pK(a)s and upon proper substitution can even form strong acids. This study employs density functional theory to estimate the gas-phase acidities (Delta G degrees s) of these compounds and applies a quantitative structure-activity relationship (QSAR) method at the B3LYP/6-31+G** level with the CPCM aqueous solvent model to estimate their aqueous pK(a)s. For the latter study, the energy difference Delta E(H2O) in water between the parent compounds and their dissociation products was used as a single parameter. Good quality QSAR regression equations were obtained for both the gas-phase (R-2 = 0.9905) and aqueous (R-2 = 0.9647) dissociations. These equations should be useful for the estimation of missing pK(a)s for compounds in this class. A general discussion of the features affecting the pK(a)s of these compounds is also given.