Ion-molecule reactions within cluster

Abstract

Proton transfer reactions that proceed within methanol cluster ions have been studied using an electron impact time-of-flight mass spectrometer. When $CH_3OH$ seeded in helium is expanded, the protonated species, $(CH_3OH)_nH^+$, are the predominant cluster ions in the low mass region. In $CH_3OD$ clusters, both $(CH_3OD)_nH^+$ and $(CH_3OD)_nD^+$ ions are observed. The ion abundance ratio, $(CH_3OD)_nH^+$/$(CH_3OD)_nD^+$, decreases as the methanol concentration increases, which is apparently related to the cluster structure and reaction energetics. The results suggest that the effective formation of $(CH_3OD)_nH^+$ ions at low concentration of $CH_3OD$ is responsible for the feasible rotation of methanol molecule within the cluster ions. Ab initio molecular orbital calculations have been carried out to investigate the rearrangement and dissociative pathways of ionized methanol dimer. Ion-neutral complexes [$CH_3OH_2^+\ldots{O}(H)$CH_2$] and [$CH_3OH_2^+\ldots{OCH}_3$] are found to play an important role in low energy pathways for the production of $CH_3OH_2^+$ + $CH_2OH$ (or $CH_3O$) from ionized methanol dimer. The reaction energetics of proposed mechanisms are compared with expeimental measurements. Chemical behaviors of $CH_3OH$ within acetylene/methanol and ethylene/methanol heteroclusters have been studied using electron-impact time-of-flight mass spectrometer. The effects of both mixing counterpart and mixing ratio upon the observed methanol cluster ions have been examined. In the case of acetylene/methanol heterocluster, the ion intensity ratio of $CH_3OH^+$/$CH_2OH^+$ increases as the concentration of methanol component decreases. When the trace amount of methanol presents, methanol in acetylene/methanol cluster ionizes by charge transfer from the initially ionized acetylene as quite contrary to the direct ionization of methanol molecule in the ethylene/methanol cluster. The abundace ratio, $CH_3OH_2^+$/$CH_3OH^+$, increases as the ethylene/methanol mi...