CONCERTED BIPROTONIC TAUTOMERISM OF 2-HYDROXYPYRIDINE

Theoretical ab initio calculations have been performed to estimate the potential energy barrier for a concerted biprotonic tautomerization i n 2-hydroxypyridine cyclic dimers. At the SCF/(mixed basis set) level these calculations suggest that this barrier is about 60 kJ mol-1. Thi s 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 th e Rice-Ramsperger-Kassel-Marcus theory. The numerical solution of the corresponding kinetic equations (with estimated rate constants) sugges ts that the concerted biprotonic tautomerization of 2-hydroxypyridine gaseous samples may occur within 4 x 10(-5) to 4 x 10(-6) s. That shou ld be fast enough to facilitate the tautomeric equilibrium to be achie ved in the course of the flight of molecules during deposition of low- temperature matrices. Tautomeric processes involving concerted transfe r 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 e xperiments.