金属正离子与甲烷的反应活性有着广泛的研究，但已报道的离子分子反应通常在单次碰撞或低碰撞能条件下发生。本文介绍最近搭建的一个由迁移管与离子漏斗组成的离子分子反应装置，在多次碰撞与可变碰撞能条件下研究离子分子反应。离子源产生的Au+在迁移管内与甲烷反应，经漏斗与离子阱收集后由质谱检测。该反应装置的反应气压可达100 Pa，且离子与分子的碰撞能可通过迁移管与离子漏斗间的电势差调节。利用该装置，我们研究了闭壳层Au+离子与甲烷的反应，并观测到碳―碳偶联产物AuC2H4+生成。密度泛函理论计算表明经由Au―CH2与Au―CH3物种的两条反应通道均可发生碳―碳偶联反应(Au+ + 2CH4→ AuC2H4+ + 2H2)。离子轨迹模拟表明迁移管与漏斗间的电场可提供足够的碰撞能促进碳―碳偶联反应发生。
The reactivity of atomic metal cations toward CH4 has been extensively investigated over the past decades. Closed-shell metal cations in electronically ground states are usually inert with CH4 under thermal collision conditions because of the extremely high stability of methane. With the elevation of collision energies, closed-shell atomic gold cations (Au+) have been reported to react with CH4 under single-collision conditions to produce AuCH2+, AuH+, and AuCH3+ species. Further investigations found that the ion-source-generated AuCH2+ cations can react with CH4 to synthesize C―C coupling products. These previous studies suggested that new products for the reaction of Au+ with CH4 can be identified under multiple-collision conditions with sufficient collision energies. However, the reported ion-molecule reactions involving methane were usually performed under single- or multiple-collision conditions with thermal collision energies. In this study, a new reactor composed of a drift tube and ion funnel is constructed and coupled with a homemade reflectron time-of-flight mass spectrometer. Laser-ablation-generated Au+ ions are injected into the reactor and drift 120 mm to react with methane seeded in the helium drift gas. The reaction products and unreacted Au+ ions are focused through the ion funnel and accumulate through a linear ion trap and are then detected by a mass spectrometer. In the reactor, the pressure is approximately 100 Pa, and the electric field between the drift tube and ion funnel can regulate the collision energies between ions and molecules. The reaction of the closed-shell atomic Au+ cation with CH4 is investigated, and the C―C coupling product AuC2H4+ is observed under multiple-collision conditions with elevated collision energies. Density functional theory calculations are performed to understand the mechanism of the coupling reaction (Au++ 2CH4 → AuC2H4+ + 2H2). Two pathways involving Au―CH2 and Au―CH3 species can separately mediate the C―C coupling process. The activation of the second C―H bond in each process requires additional energy to overcome the relatively high barrier (2.07 and 2.29 eV). Ion-trajectory simulations under multiple-collision conditions are then conducted to