The energies, pK(Enol) values, structures, and conformations of vinyl alcohol (3a), acetaldehyde (4a), 1,1-ethenediol (5a), acetic acid (6a), and six derivatives of each (3b-g, 4b-g, 5b-h, and 6b-h) substituted by two beta -aryl groups of increasing bulk from Ph to Tip (2, 4,6-tri-i-PrC6H2) were calculated by the B3LYP method in order to evaluate quantitatively the pK(Enol) reducing effect of bulky aryl groups. Also calculated were the same parameters for the ArCHXCO2H/ArC(X)=C(OH)(2) pairs for Ar = Ph, X = CN (9a/9b), OH (10a/10b), for which pK(Enol) is known, and for Ar = Mes (mesityl), X = CN. All the substituents significantly decrease the pK(Enol) values from 22.4 (6a/5a) and 9.1 (4a/3a). For example, pK(Enol) = 0.2, -2.7, -2.0, and -2.7 for ArA'CHCHO/ArAr'C=CHOH and 13.3, 11.5, 9.2, and 9.3 for ArAr'CHCO2H/ArAr'C=C(OH)(2) when Ar,Ar' = Ph,Ph, Ph,Mes, Mes,Mes, and Tip,Tip, respectively. For 9a/9b and POa/lOb, the calculated pK(Enol) values resemble the observed ones. The enols and enediols (except 3b and 5b, when Ar2C= = fluorenylidene) have a propeller conformation. The Ar-C=C torsional angles increase with increasing bulk of the aryl group, but for Ph,bulkier Ar the Ar-C=C torsional angle strongly exceeds the Ph-C=C angle. The syn-C=C-O-H conformer is preferred over the anti conformer for all the enols 3. Far the 1,1-enediols, the syn.syn conformer is preferred for most Ar,Ar' combinations, but the syn,anti conformer is preferred for 5a. Sb, 5h (Ar = Ph, Ar' = 2,4,6-(t-Bu)(3)C6H2), and 10a, and the anti,anti conformation is the least stable. The stabilization and conformational preferences were analyzed both qualitatively and with the aid of appropriate isodesmic reactions. Superposition of stabilizing Ar-C=C conjugation effects, stabilization of the carbonyl forms, and pi>(*) over bar * (Ar). . .O hydrogen bonding and destabilizing geminal Ar/Ar' and vicinal cis-Ar/OH steric interactions account for the results. The low enol or enediol content is mainly due to the relative stabilization of the aldehyde or acid form, and the beta -Ar groups stabilize the enols mainly by conformation-dependent Ar C=C conjugation and pi>(*) over bar * (Ar). . . HO H-bonding.