Proton transfer in functionalized phosphonic acid molecules.
Proton transfer in functionalized phosphonic acid molecules.
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DOI：
10.1039/b917903h
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发表时间：
2010-01
期刊：
影响因子：
--
通讯作者：
Chen Wang;S. Paddison
中科院分区：
文献类型：
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作者：
Chen Wang;S. Paddison
具有原生基团的离聚体，如膦酸，由于其非凡的两性性，最近被提出作为适用于很少加湿的燃料电池的聚合物电解质膜。通过从头算电子结构计算，研究了取代膦酸分子及其自缩合产物（酸酐+ H(2)O）的氢键和质子转移相关的能量学。在B3LYP/6-311G**水平上测定的甲基-、苯基-、苄基-、三氟甲基-和苯基二氟膦酸的全局最小能量结构表明，氟化分子与水的结合比非氟化分子略强。在B3LYP/6-311G**水平上得到了缩合产物中质子转移的势能谱，结果表明三氟甲基体系具有最低的吸热性（5.3 kcal mol(-1)），而甲基体系具有最大的吸热性（7.1 kcal mol(-1)）和7.6 kcal mol（-1）的势垒。当系统不受任何约束时，除了三氟甲基酸酐发生电荷分离和水合氢离子形成外，所有情况下，质子从酸酐转移到水分子（伯水分子）都伴随着二次质子转移（从质子化的水分子回到酸酐）。二级质子转移似乎在很大程度上决定了过渡态的出现和相关的吸热度。
Ionomers with protogenic groups such as phosphonic acid have recently been proposed as suitable polymer electrolyte membranes for fuel cells operating with little humidification due to their extraordinary amphotericity. The hydrogen bonding and energetics associated with proton transfer of substituted phosphonic acid molecules and their self-condensation products (anhydride + H(2)O) are examined through ab initio electronic structure calculations. The global minimum energy structures of methyl-, phenyl-, benzyl-, trifluoromethyl- and phenyldifluoro-phosphonic acids determined at the B3LYP/6-311G** level indicate that the fluorinated molecules exhibit slightly stronger binding to water than the non-fluorinated molecules. The potential energy profiles for transfer of a proton within the condensation products were obtained at the B3LYP/6-311G** level for each of the acids, and revealed that the trifluoromethyl system has the lowest endothermicity (5.3 kcal mol(-1)) in contrast to the methyl system which has the greatest endothermicity (7.1 kcal mol(-1)) and a barrier of 7.6 kcal mol(-1). When no constraints are imposed on the system, proton transfer from the anhydride to a water molecule (primary) is accompanied by a secondary proton transfer (from protonated water molecule back to the anhydride) in all cases except the trifluoromethyl anhydride where charge separation and formation of a hydronium ion occurs. It appears that the secondary proton transfer would substantially determine the emergence of transition states and the correlated endothermicities.