ACIDITIES OF ALCOHOLS AND CARBOXYLIC-ACIDS - EFFECT OF ELECTRON CORRELATION

The question of the acidity of carboxylic acids relative to alcohols is investigated at a higher level of theory than has been done previously. Formic acid and methanol are used as examples. It is shown through the Hellmann-Feynman theorem that the potential at the acidic proton can be evaluated from the derivative of total energy with respect to proton charge. Using this procedure, the potential energy of the acidic proton in these molecules has been evaluated at the MP2/6-311++G(2d,p) level. Combining these potential energies with acidities calculated at the same level gives the contribution of relaxation in the anion to the acidity. In agreement with earlier, lower level, calculations, these calculations show that the principal factor contributing to the higher acidity of formic acid is the initial-state potential energy of the acidic proton. Relaxation in the anion, including stabilization due to the resonance, contributes little to the acidity difference.