Nuclear magnetic resonance and ab initio studies of small complexes formed between water and pyridine derivatives in...

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 Angstrom for pyridine and 1.92 Angstrom 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 pK(a) 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 Angstrom for pyridine and 1.64 Angstrom 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 pK(a) 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 water molecules are involved. Thus, the proton-donating ability of the outer hydroxyl groups of biologically relevant water bridges should be comparable to the proton-donating ability of a fictitious acid exhibiting a pK(a) value of about 4.9 in water. Driven by the mixing entropy, monomeric water presented in the aprotic freonic mixtures above 170 K but completely precipitated upon further cooling. Traces of water could be suspended in the mixtures down to 130 K in the presence of about 20-fold excess of heterocyclic bases. The obtained experimental data indicated that at these conditions water trended to form the symmetric 2:1 heterocycle-water complexes, whose bridge protons resonated around 6.7 ppm.