Origin of the Inversion of the Acidity Order for Haloacetic Acids on Going from the Gas Phase to Solution

The change in the relative acidities of the haloacetic acids on going from the gas phase to aqueous solution has been studied via ab initio calculations for the gas phase, the SCIPCM reaction field model for an aprotic polar solvent, and Monte Carlo statistical mechanics for aqueous solution. The relative gas phase acidities of acetic acid, the haloacetic acids and trifluoroacetic acid are well reproduced at the B3P86/6-311+G** and G2 levels of theory. The reaction field model reduced the relative acidities of fluoro- and bromoacetic acids to a value close to that found in aqueous solution, but it only reproduced half of the observed net effect of going from the gas phase to aqueous solution. Since the remainder of the solvent effect was probably due to hydrogen bonding, Monte Carlo calculations were carried out and they were in satisfactory accord with the aqueous solution pKa's.