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

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• 作者：Shasad Sharif ; Ilja G. Shenderovich ; Leticia Gonzá ; lez ; Gleb S. Denisov ; David N. Silverman ; Hans-Heinrich Limbach
• 刊名：Journal of Physical Chemistry A
• 出版年：2007
• 出版时间：July 12, 2007
• 年：2007
• 卷：111
• 期：27
• 页码：6084 - 6093
• 全文大小：324K
• 年卷期：v.111,no.27(July 12, 2007)
• ISSN：1520-5215

文摘

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 spectroscopyin 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 ¹⁵NNMR. When base was in excess, one water molecule was symmetrically bonded to two heterocyclic molecules.This complex was characterized by the r_HN distances of 1.82 Å for pyridine and 1.92 Å for collidine. Theproton-donating ability of water in such complexes was affected by an anticooperative interaction betweenthe 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 modelheterocycle-water clusters indicated that water in such clusters was oriented as a chain. The NMR estimatedr_HN distances in these species were 1.69 Å for pyridine and 1.64 Å for collidine. Here, the proton-donatingability of the hydroxyl group bonded to the heterocycle was affected by a mutual cooperative interactionwith other water molecules in the chain and became comparable to the proton-donating ability of a fictitiousacid, exhibiting an apparent pK_a value of about 4.9. This value seems to depend only slightly on the lengthof the water chain and on the presence of another base at the other end of the chain if more than two watermolecules are involved. Thus, the proton-donating ability of the outer hydroxyl groups of biologically relevantwater bridges should be comparable to the proton-donating ability of a fictitious acid exhibiting a pK_a valueof about 4.9 in water. Driven by the mixing entropy, monomeric water presented in the aprotic freonic mixturesabove 170 K but completely precipitated upon further cooling. Traces of water could be suspended in themixtures down to 130 K in the presence of about 20-fold excess of heterocyclic bases. The obtainedexperimental 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.