Proton affinity of para-substituted acetophenones in gas phase and in solution: a theoretical study

The gas phase proton affinities PA and basicities GB for a series of para-substituted acetophenones weak bases (B) p.X???C₆H₄CO*CH₃ with X?=?H, F, Cl, Br, I, Me, CF₃, CN, NO₂, OCH₃, NH₂, CH₂OH, N(CH₃)₂, OH, $ {text{NH}}_3^{+} $ , … have been calculated at 298.15 K at the density functional theory DFT/B3LYP level with a 6-311++G (2d,2p) basis set. Conformational results lead to only one stable planar conformer for both unprotonated compounds and their O*-protonated forms. Satisfactory accuracy and computational efficiency could be reached if the computed PAs are scaled by a factor 0.983. Protonation at more than one site is discussed and the carbonyl oxygen atom is found to be the preferential protonated site rather than the substituent X. The calculated gas phase PAs show a good agreement with the experimental available data. The electron-donating/electron-withdrawing nature of the substituents has an enormous influence upon the thermochemical and structural properties. The influence of environment on the proton affinity has been studied by means of SCRF solvent effect computations using PCM solvation model for two solvents: water and SO₂CI₂. Confrontation between computed and experimental pK(B) values exhibits better agreement in aqueous solution than in organic solvent.