Wang Yi, Zhang Xinxing, Lyapustina Svetlana, Nilles Michael M, Xu Shoujun, Graham Jacob D, Bowen Kit H, Kelly John T, Tschumper Gregory S, Hammer Nathan I
Department of Chemistry, Johns Hopkins University, Baltimore, MD 21218, USA.
Department of Chemistry and Biochemistry, University of Mississippi, University, MS 38677, USA.
Phys Chem Chem Phys. 2016 Jan 14;18(2):704-12. doi: 10.1039/c5cp02746b.
The prospect that protons from water may be transferred to N-heterocyclic molecules due to the presence of an excess electron is studied in hydrated azabenzene cluster anions using anion photoelectron spectroscopy and computational chemistry. In the case of s-triazine (C3H3N3), which has a positive adiabatic electron affinity, proton transfer is not energetically favored nor observed experimentally. Heterocyclic rings with only 1 or 2 nitrogen atoms have negative electron affinities, but the addition of solvating water molecules can yield stable negative ions. In the case of the diazines (C4H4N2: pyrazine, pyrimidine, and pyridazine) the addition of one water molecule is enough to stabilize the negative ion, with the majority of the excess electron density in a π* orbital of the heterocycle but not significantly extended over the hydrogen bonded water network. Pyridine (C5H5N), with the most negative electron affinity, requires three water molecules to stabilize its negative ion. Although our computations suggest proton transfer to be energetically viable in all five N-heterocyclic systems studied here when three or more water molecules are present, proton transfer is not observed experimentally in the triazine nor in the diazine series. In pyridine, however, proton transfer competes energetically with hydrogen bonding (solvation), when three water molecules are present, i.e., both motifs are observed. Pyridine clusters containing four or more water molecules almost exclusively exhibit proton transfer along with solvated C(6-x)H(6-x+1)N(x)·OH ions.
利用阴离子光电子能谱和计算化学方法，在水合氮杂苯簇阴离子中研究了由于存在过量电子，水中的质子可能转移到氮杂环分子上的前景。对于具有正绝热电子亲和能的均三嗪（C3H3N3），质子转移在能量上并不有利，实验中也未观察到。仅含有1个或2个氮原子的杂环具有负电子亲和能，但加入溶剂化水分子可产生稳定的负离子。对于二嗪（C4H4N2：吡嗪、嘧啶和哒嗪），加入一个水分子就足以使负离子稳定，大部分过量电子密度位于杂环的π*轨道中，但在氢键水网络中没有显著扩展。具有最负电子亲和能的吡啶（C5H5N）需要三个水分子来稳定其负离子。虽然我们的计算表明，当存在三个或更多水分子时，本文研究的所有五个氮杂环系统中质子转移在能量上是可行的，但在均三嗪和二嗪系列中实验上未观察到质子转移。然而，在吡啶中，当存在三个水分子时，质子转移在能量上与氢键（溶剂化）相互竞争，即两种模式都被观察到。含有四个或更多水分子的吡啶簇几乎只表现出质子转移以及溶剂化的C(6 - x)H(6 - x + 1)N(x)·OH离子。
