Catalyzed ethen-1,1-diol-acetic acid tautomerizations in gas phase by H2O, NH3, H2S, HCl, and HF were studied by ab initio calculations. Corresponding ethenol-acetaldehyde tautomerizations were also studied for comparison purposes. According to an isodesmic reaction calculated at the level of MP2/6-31G*, the second hydroxyl group on ethen-1,1-diol provides 4.5 kcal/mol more thermodynamic stability than ethenol. As acetic acid has a much higher stability than acetaldehyde, it makes pK(E) of ethen-1,1-diol bigger than that of ethenol. Transition states of catalyzed keto-enol tautomerizations were located and all the catalyzed tautomerizations found to involve a concerted mechanism. As far as catalyzed and uncatalyzed keto-enol tautomerizations were concerned, ketonization of ethen-1,1-diol has a lower activation energy than that of ethenol while enolization of acetic acid has a higher activation energy than that of acetaldehyde. Both H2O and NH3 function as base catalysts in the tautomerizations, while H2S, HCl, and HF function as acid catalysts. Regardless of acidic or basic catalysts, the catalytic effect on ethen-1,1-diol-acetic acid tautomerization is more efficient than that on ethenol-acetaldehyde tautomerization by ca. 2 kcal/mol.
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