Proton transfer reaction and intermolecular interactions in associates of 2,5-dihydroxy-1,8-naphthyridine.

Tautomerism in monomers/dimers and association of 2,5-dihydroxy-1,8-naphthyridine was studied at the DFT level recently recommended for studies of non-covalent interactions. Studied dimers are stabilized by double and triple hydrogen-bonding. In some associates the intermolecular proton transfer may take place. Transition state related to the double proton transfer reactions were calculated and discussed in terms of energetics, changes in atomic charges upon association, aromaticity (HOMA), properties of hydrogen bond critical point (QTAIM methodology) and geometry change during this reaction. It was found that double proton transfer is supported by third hydrogen bond or by weak secondary interaction. Some protons in transition states are shared between two basic atoms, while other are covalently bound only to one of them. The said process leads to replacement of secondary interactions of attractive character to repulsive and vice versa. Overall, results suggest that in subjected compound the triple hydrogen-bonded associate may be in equilibrium with double hydrogen-bonded dimer.