Ahlberg P, Karlsson A, Goeppert A, Nilsson Lill S O, Dinér P, Sommer J
Department of Chemistry, Göteborg University, Sweden.
Chemistry. 2001 May 4;7(9):1936-43. doi: 10.1002/1521-3765(20010504)7:9<1936::aid-chem1936>3.0.co;2-t.
The transition states for methane activation in liquid superacid have been studied by experimentally determined secondary kinetic deuterium isotope effects (SKIEs) and computational chemistry. For the first time, the SKIEs on hydrogen/deuterium exchange of methane have been measured by using the methane isotopologues in homogeneous liquid superacid (2HF/SbF5). To achieve high accuracy of the SKIEs, the rate constants for pairs of methane isotopologues were simultaneously measured in the same superacid solution by using NMR spectroscopy. Density functional theory (DFT) and high-level ab initio methods have been employed to model possible intermediates and transition states, assuming that the superacids involved in the exchange reactions are H2F+ ions solvated by HF. Only the unsolvated superacid H2F+ is found to be strong enough to protonate methane, yielding the methonium ion solvated by HF as a potential energy minimum. In contrast, the (HF)x-solvated H2F+ superacids (x = 1-4) do not appear to be strong enough to yield stable solvated methonium ions. However, such ions show up as parts of the transition states of the exchange in which the methonium ions are solvated by (HF)x. The calculated DFT activation barrier is in good agreement with that experimentally observed.
通过实验测定的二级动力学氘同位素效应（SKIEs）和计算化学方法，对液态超强酸中甲烷活化的过渡态进行了研究。首次利用均相液态超强酸（2HF/SbF5）中的甲烷同位素异构体，测量了甲烷氢/氘交换的SKIEs。为了实现SKIEs的高精度测量，采用核磁共振光谱法在同一超强酸溶液中同时测量了甲烷同位素异构体对的速率常数。采用密度泛函理论（DFT）和高水平的从头算方法，对可能的中间体和过渡态进行建模，假设参与交换反应的超强酸是由HF溶剂化的H2F+离子。结果发现，只有未溶剂化的超强酸H2F+足够强，能够使甲烷质子化，生成由HF溶剂化的甲鎓离子作为势能最小值。相比之下，（HF）x溶剂化的H2F+超强酸（x = 1-4）似乎不够强，无法生成稳定的溶剂化甲鎓离子。然而，这些离子作为交换过渡态的一部分出现，其中甲鎓离子由（HF）x溶剂化。计算得到的DFT活化能垒与实验观察结果吻合良好。
