Abstract
Two tautomeric forms of a heterocyclic monomer -the preferable M-0 and the one at higher energy, M*, can form three different kinds of hydrogen-bonded dimers: (MM0)-M-0, (MM)-M-0* and M* M*. The lowest-energy dimer (MM0)-M-0 and the highest one M* M* are transformed into each other by double proton transfer (DPT). By the corresponding concerted process, the mixed complex (MM)-M-0* is converted to the equivalent isomer M*M-0. It cannot return to the low-energy form (MM0)-M-0 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 ( 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.
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