Reactions of Al+ and Mg+ with Acetonitrile in Gas Phase Using Laser Ablation-Molecular Beam Method

The gas phase reactions of Al+ and Mg+ with acetonitrile are studied by laser ablation-molecular beam method. According to the results of reflectron flight time mass spectrometer (RTOF-MS) , Al+ and Mg+ react with acetonitrile to form different sized cluster ion products. A series of cluster complex ions Al+(CHCN)n(n=1~10) can be found and cluster complex ions Mg+(CHCN)n(n=1-5) are produced. The appearances of the complex ions Al+ (CHCN)n and Mg+(CHCN)n is obviously sensitive to the kind and pressure of rare gas used for the seeded acetonitrile beams. The phenomenon can be explained by the γ effect and the thermal accommodation coefficient. In laser ablation molecular beam experiments, Ar has a larger γ effect and a larger accommodation coefficient than He, so the former is Favor able for acetonitrile cluster growth. The energy of laser beams is an important factor which influences the size of clustered complex ions. According to the result of experiments, the energy of laser beams which can be controlled around or less than 2 mJ/pulse was favorable for forming clustered complex ions. Generally, decreasing laser energy can generate low energy metal ions. Although metal ions have rather wide kinetic energy (KE) distribution in LAMB experiments, only the low-energy portions of metal ions are effective in the reactions in their experimental configuration with the M+ flow and the molecular beam perpendicular to each other. The signal intensities of clustered complex ions Al+(CHCN)n and Mg+(CHCN)n show irregular distribution. For Al+(CHCN)n, the first intensity gap appears between n=4~5, and the second intensity gap is found between n=6~7. But for Mg+(CHCN)n, the intensity gap appear between n=2~3. The intensity gaps of Al+(CHCN)n and Mg+ (CHCN)n are relative to the binding energy of metal cation-orginic ligands. It is believed that the binding energy between Al+ and the fourth CHCN ligand is obviously higher than that between Al+ and the fifth CHCN ligand. Therefore, the intensity gap found in the present study indicates that the first coordination sphere for Al+ is completed at n=4. The fifth CHCN ligand and the sixth CHCN ligand must be loosely coordinated with the central Al+. Association reactions and dehydrogenation reactions are the two principal reaction pathways when metal ions react with organic ligands in LAMB experiments. When Al+ and Mg+ interact with (CHCN)n(n≥3), only the clustered complex ions Al+(CHCN)n and Mg+(CHCN)n can be formed through association pathways. However, when Al+ and Mg+ coordinate with one acetonitrile or two acetonitriles, not only Al+(CHCN)n(n≤2) and Mg+(CHCN)n(n≤2) can be produced through association reactions, but also Al+(CH2CN)n(n≤2) and Mg+(CH2CN)n(n≤2) can be formed through dehydrogenation reactions.