These studies illustrate the importance of both polarity of reactants and hydrogen bonding ability in kinetic solvent effects of Diels-Alder reactions, and verify that continuum models and other computational methods can capture both of these effects reasonably. Despite the success of these models, experimental studies were necessary to show the hydrophobic effect that is crucial in some cases. Additionally, merely adding solvation energies to those obtained in the gas-phase is not sufficient when the transition state geometries or shape of the potential energy surface change significantly in a solvent. Xu et al. theoretically investigated the catalyst-assisted acetylation of tertbutanol, displayed in Figure with acetic anhydride in the gas phase and in three solvents. The optimized geometries and reaction energies were found with B3LYP/6-311++G(d,p) and B3LYP/6-31G(d), respectively, while solvation energies were found using PCM. The calculations showed that the reaction proceeds via a mechanism characterized by nucleophilic attack of the catalysis, and that this mechanism does not change, nor does the ratelimiting step, with solvent. Because polar solvents better solvate the reactants than the transition states or reaction intermediates, the reaction is less favorable in these solvents. Consequently, the reaction proceeds less favorably in going from gas, to carbon tetrachloride, to chloroform, and finally to dichloromethane, the most polar solvent of the group. These results are also supported by early experimental studies . The investigations together illustrate that continuum solvation models can be sufficient to explain trends in solvation kinetics, and also that reaction rate can vary with solvent polarity alone.