Li Feng-Xia, Armentrout P B
Chemistry Department, University of Utah, Salt Lake City, Utah 84112, USA.
J Chem Phys. 2006 Oct 7;125(13):133114. doi: 10.1063/1.2220038.
The potential energy surface for activation of methane by the third-row transition metal cation, Au+, is studied experimentally by examining the kinetic energy dependence of this reaction using guided ion beam tandem mass spectrometry. A flow tube ion source produces Au+ primarily in its 1S0 (5d10) electronic ground state level but with some 3D (and perhaps higher lying) excited states that can be completely removed by a suitable quenching gas (N2O). Au+ (1S0) reacts with methane by endothermic dehydrogenation to form AuCH2+ as well as C-H bond cleavage to yield AuH+ and AuCH3+. The kinetic energy dependences of the cross sections for these endothermic reactions are analyzed to give 0 K bond dissociation energies (in eV) of D0(Au+ - CH2) = 3.70 +/- 0.07 and D0(Au+ -CH3) = 2.17 +/- 0.24. Ab initio calculations at the B3LYPHW + /6-311++G(3df,3p) level performed here show good agreement with the experimental bond energies and previous theoretical values available. Theory also provides the electronic structures of the product species as well as intermediates and transition states along the reactive potential energy surface. Surprisingly, the dehydrogenation reaction does not appear to involve an oxidative addition mechanism. We also compare this third-row transition metal system with the first-row and second-row congeners, Cu+ and Ag+. Differences in thermochemistry can be explained by the lanthanide contraction and relativistic effects that alter the relative size of the valence s and d orbitals.
通过使用导向离子束串联质谱法研究该反应的动能依赖性，对第三周期过渡金属阳离子Au⁺活化甲烷的势能面进行了实验研究。流动管离子源产生的Au⁺主要处于其1S₀(5d¹⁰)电子基态能级，但也有一些3D（可能还有更高能级）激发态，这些激发态可通过合适的猝灭气体（N₂O）完全去除。Au⁺(1S₀)与甲烷通过吸热脱氢反应形成AuCH₂⁺，以及通过C-H键断裂生成AuH⁺和AuCH₃⁺。分析了这些吸热反应截面的动能依赖性，得到0K时的键解离能（单位为eV）：D₀(Au⁺-CH₂)=3.70±0.07和D₀(Au⁺-CH₃)=2.17±0.24。在此处B3LYPHW+/6-311++G(3df,3p)水平上进行的从头算计算与实验键能和现有的先前理论值显示出良好的一致性。理论还提供了产物物种以及沿反应势能面的中间体和过渡态的电子结构。令人惊讶的是，脱氢反应似乎不涉及氧化加成机制。我们还将这个第三周期过渡金属体系与第一周期和第二周期的同族元素Cu⁺和Ag⁺进行了比较。热化学上的差异可以用镧系收缩和相对论效应来解释，这些效应改变了价s和d轨道的相对大小。
