Nuclear Magnetic Resonance and ab Initio Studies of Small Complexes Formed between Water and Pyridine Derivatives in Solid and Liquid Phases

The structure and geometry of hydrogen-bonded complexes formed between heterocyclic bases, namely, pyridine and 2,4,6-trimethylpyridine (collidine), and water were experimentally studied by NMR spectroscopy in frozen phase and in highly polar aprotic liquefied freon mixtures and theoretically modeled for gas phase. Hydrogen-bonded species in frozen heterocycle-water mixtures were characterized experimentally using 15N NMR. When base was in excess, one water molecule was symmetrically bonded to two heterocyclic molecules. This complex was characterized by the rHN distances of 1.82 Å for pyridine and 1.92 Å for collidine. The proton-donating ability of water in such complexes was affected by an anticooperative interaction between the two coupled hydrogen bonds and exhibited an apparent pKa value of about 6.0. When water was in excess, it formed water clusters hydrogen bonded to base. Theoretical analysis of binding energies of small model heterocycle-water clusters indicated that water in such clusters was oriented as a chain. The NMR estimated rHN distances in these species were 1.69 Å for pyridine and 1.64 Å for collidine. Here, the proton-donating ability of the hydroxyl group bonded to the heterocycle was affected by a mutual cooperative interaction with other water molecules in the chain and became comparable to the proton-donating ability of a fictitious acid, exhibiting an apparent pKa value of about 4.9. This value seems to depend only slightly on the length of the water chain and on the presence of another base at the other end of the chain if more than two wate