Origin of the inversion of the acidity order for haloacetic acids on going from the gas phase to solution

An examination is done of the change in the relative acidities of the halocetic acids on going from the gas phase to aqueous solution which 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. 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 pK(sub a)'s.