Less stable tautomers form stronger hydrogen bonds: the case of water complexes

Hydrogen bonding in cyclic complexes of water with tautomeric pairs of molecules M⁰ and M¹ is calculated to be stronger by more than 25% for the less stable tautomer M¹ in all cases where the energy gap between the two tautomers is large (ΔE(M⁰ - M¹) > 10 kcal mol^-1). This is accompanied by a large red-shift (>200 cm^-1) of the N-H/O-H stretch frequency in the complexes involving M¹. Large barriers for double proton transfer in both directions should permit an experimental verification. Exceptions to this rule were found in heterocycles with an N-C[double bond, length as m-dash]O fragment incorporated into a conjugated cycle resulting in two nearly degenerate tautomers - keto and enol forms. The wavefunction of the keto form has a large contribution from a zwitterionic VB structure which is also aromatic. This increases the polarity of the keto group, making the oxygen atom a strong H-bond acceptor. It can also stabilize the keto form below the aromatic enol form. In this case the extra-HB stabilization is observed for the most stable tautomer (i.e. for the keto form). H-bonding enhances the aromatic character of less aromatic molecules, but the more aromatic tautomers partially loose aromaticity.