Finding structural principles for strong hydrogen–bonds: Less stable tautomers form dimers with stronger hydrogen bonds.

Two tautomeric forms of a heterocyclic monomer – the preferable M⁰ and the one at higher energy, M*, can form three different kinds of hydrogen-bonded dimers: M⁰M⁰, M⁰M* and M*M*. The lowest-energy dimer M⁰M⁰ and the highest one M*M* are transformed into each other by double proton transfer (DPT). By the corresponding concerted process, the mixed complex M⁰M* is converted to the equivalent isomer M*M⁰. It cannot return to the low-energy form M⁰M⁰ without breaking the hydrogen bonds. A quantum-chemical study of dimers of tautomeric monomers (7-azaindole, 1-azacarbazole, formamide) and non-symmetric complexes including the DNA adenine-thymine pair shows increasing hydrogen bond stabilization with increasing energy of the dimer or complex, respectively. The least stable dimer M*M* has a small barrier for reverse DPT (< 2 kcal mol⁻¹), rendering an experimental observation difficult. This should not be the case for the mixed complex M⁰M* which is kinetically stable (▵E^≠ > 8 kcal mol⁻¹). Hence the mixed dimer is a perspective candidate for the experimental verification of hydrogen bond-stabilization in less stable tautomers. Theory predicts softening of the OH/NH bonds and a strong down shift of the OH/NH frequencies by > 200 cm⁻¹ in the less stable complexes.