How do strong hydrogen bonds affect the acidities of carbon acids? An ab initio molecular orbital study

The deprotonation energies and proton affinities of acetaldehyde have been determined using ab initio molecular orbital calculations at the MP2/D95** level for the parent molecules and the hydrogen bonding dimers formed from a keto and an enol tautomer. The C-H deprotonation energies of acetic acid and its hydrogen-bonded dimer have been similarly determined, In all cases, the deprotonation energy is greatly enhanced in the dimeric form. The proton affinity is likewise enhanced for two cases studied. The current results suggest that enhancements of both acidities and basicities of 20-30 kcal/mol might be expected in enzymes where such hydrogen bonds are possible.