Concerted biprotonic tautomerism of 2-hydroxypyridine

Theoretical ab initio calculations have been performed to estimate the potential energy barrier for a concerted biprotonic tautomerization in 2-hydroxypyridine cyclic dimers. At the SCF/(mixed basis set) level these calculations suggest that this barrier is about 60 kJ mol^-1 . This value is about three times smaller than the potential energy barrier for a single proton transfer in monomers. The rate constants for the tautomeric form change in dimers have also been calculated based on the Rice- Ramsperger-Kassel-Marcus theory. The numerical solution of the corresponding kinetic equations (with estimated rate constants) suggests that the concerted biprotonic tautomerization of 2-hydroxypyridine gaseous samples may occur within 4 × 10^-5 to 4 × 10^-6 s. That should be fast enough to facilitate the tautomeric equilibrium to be achieved in the course of the flight of molecules during deposition of low-temperature matrices. Tautomeric processes involving concerted transfer of two protons in the 2-hydroxypyridine cyclic dimers bonded by two hydrogen bonds seem to provide the most important way of reaching the tautomeric equilibria observed in the gas-phase and matrix-isolation experiments.