Origin of the inversion of the acidity order for haloacetic acids on goingfrom 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 calculatio ns for the gas phase, the SCIPCM reaction field model for an aprotic polar solvent, and Monte Carlo statistical mechanics far aqueous solution. The re lative gas phase acidities of acetic acid, the haloacetic acids and trifluo roacetic acid are well reproduced at the B3P86/5,311+G** and G2 levels of t heory. The reaction field model reduced the relative acidities of fluoro- a nd bromoacetic acids to a value close to that found in aqueous solution, bu r 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 pr obably due to hydrogen bonding, Monte Carlo calculations were carried out a nd they were in satisfactory accord with the aqueous solution pK(a)'s.