Dynamics of C-C coupling reactions of activated carbon centers by crossed beam imaging

碳-碳偶联反应是合成过程中不可或缺的一部分，通常使用有机金属催化剂，中间活性碳中心通过与金属中心相互作用而形成。本项目的重点是两种类型的活性碳中心(1)亚烷基M-CH3和(2)卡宾M=CH2，第一种在Ziegler-Natta化学中很重要，第二种在烯烃歧化反应中很重要。尽管金属碳键的性质非常不同，但人们普遍认为，对于这两个中间体，偶联反应都会经历一个四元环过渡态。目前使用的过渡金属主要来自4d和5d系列。这些元素稀有而昂贵，因此研究试图用更丰富的3D过渡金属来取代它们，例如铁或钴，这将是我们选择的过渡金属。过渡金属化学的结果很难预测，因为金属的许多紧密的电子态可以导致状态选择性反应、非统计产物的形成或自旋表面的有效交叉。本项目的目的是用M-CH3+和M=CH2+模型体系研究气相碳键形成反应的原子动力学。射频多极离子陷阱中的原位离子形成使我们省去了凝聚相所需的任何稳定配体。我们将使用交叉束三维速度图成像来研究原子级动力学，以记录产物离子的速度分布，即微分截面。从实验数据中，我们可以提取关于原子机制的信息，这意味着原子在反应过程中如何重排，以及反应是直接的还是间接的。此外，我们了解到沿反应坐标的势垒的影响，或者反应是否跟随可能是非统计的。通过记录全速度矢量，我们可以在相同的实验中调查竞争的产品渠道，并得出分支比。碰撞能量依赖的实验将使我们对这些竞争、障碍的重要性和遭遇复合体的稳定性有更多的了解。我们将研究Ta+与甲烷偶联的基准反应，第一步是Ta=CH2+的形成，然后与第二个甲烷分子反应。我们将集中讨论四元过渡态的作用，以及尽管过渡态结构复杂，但直接反应动力学是否可能，以及是否出现共同的特征。根据所选的过渡金属，沿反应坐标的势垒可能被淹没，也可能不被淹没。它们的作用将在与M-CH3+的反应中进行研究。键插入反应是一种与复分解反应竞争的反应，因此我们将仔细研究这种竞争作为碰撞能的函数。

Carbon-carbon coupling reactions are an integral part of synthetic protocols and often make use of organometallic catalysts with intermediate reactive carbon centers being formed through interaction with a metal center. This project focuses on two types of reactive carbon centres (1) alkylidens M-CH3 and (2) carbenes M=CH2 with the first important in Ziegler-Natta type chemistry and the second in olefin metathesis. Despite the very different nature of the metal carbon bond, it is accepted that for both intermediates the coupling reaction passes through a four-membered cyclic transition state. The currently used transition metals are foremost from the 4d and 5d series. These elements are rare and expensive, thus research tries to replace them by more abundant 3d transition metals, for example iron or cobalt which will be our transition metals of choice. The outcome of transition metal chemistry is hard to predict due to the number of close lying electronic states of the metal which can for example lead to state-selective reactivity, non-statistical product formation or efficient crossing of spin-surfaces. The aim of this project is to investigate the underlying atomistic dynamics of the carbon bond forming reactions in gas phase using M-CH3+ and M=CH2+ model systems in reactions with small olefines, namely ethene and propene. In-situ ion formation in an radio frequency multipole ion trap allows us to omit any stabilizing ligands needed in condensed phase. We will use crossed beam 3D velocity map imaging to investigate the atomic-level dynamics to record product ion velocity distributions, i.e. differential cross sections. From the experimental data, we can extract information on the atomistic mechanisms, that means how atoms rearrange during the reactive encounter and if a reaction is direct or indirect. Further, we learn about the influence of barriers along the reaction coordinate or if reaction follows may be non-statistical. Recording the full velocity vector allows us to investigate competing product channels in the same experiment and derive branching ratios. Collision energy dependent experiments will give additional insight into these competitions, importance of barriers and stability of encounter complexes. We will study the benchmark reaction of methane coupling by Ta+ for which the first step is the Ta=CH2+ formation followed by a reaction with a second methane molecule. We will concentrate on the role of the four-membered transition state and if direct reaction dynamics are possible despite the complex transition state structure and if a common signature emerges. Depending on the chosen transition metal, barriers along the reaction coordinate might be submerged or not. Their role will be studied in reactions with M-CH3+. Bond insertion reactions are a competing with metathesis reactions, therefore we will carefully study this competition as function of collision energy.