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A selected-ion flow tube study of some gas-phase ion-molecule reactions of potential relevance to the chemistry of dense interstellar clouds

机译:选择离子流管研究某些与浓密星际云化学相关的气相离子分子反应

摘要

Results are reported for the studies of several systems of ion-molecule reactions of potential relevance to the chemistry of interstellar clouds. Measurements were obtained using a selected-ion flow tube operated at room temperature (300 ± 5 K) and using helium buffer gas at a pressure of 0.30 ± 0.01 Torr. The proton affinities of C₄H₂ and C₂N₂ were determined by measurement of the rate coefficients for forward and reverse proton transfer reactions involving compounds of similar proton affinity. The results obtained were P A(C₂N₂) = 674 ± 4 kJ mol-¹ and PA(C₄H₂) = 741 ± 4 kJ mol-¹: this latter quantity is significantly below the literature value, based on an earlier measurement obtained from ICR bracketing. Isomerism of the ions C₂N⁺, C₃N⁺, CHN⁺ and CH₂N⁺ was investigated, using reactivity with various neutrals to distinguish between isomers. The ions CCN⁺/CNC⁺ and CCCN⁺/c-C₃N⁺ were distinguished on the basis of their reactivity with H₂: in both instances, the isomer featuring a terminal N atom reacted rapidly while the other isomer was unreactive. Identification of the isomers HCN⁺/HNC⁺ was complicated by the occurrence of tautomerisation of HCN⁺ to the more stable isomer HNC⁺ by the mechanism of 'forth and back' proton transfer which occurred with several neutral reagents: reaction with CF₄ was subsequently used to distinguish between these isomers, since HCN⁺ reacted rapidly with CF₄ while HNC⁺ was unreactive. The reactions of all of these isomeric systems were examined with several neutrals abundant in interstellar clouds. The ions HCNH⁺ and CNH₂⁺ could not be distinguished on the basis of reactivity with the neutrals surveyed: we cannot exclude the possibility that only one of these isomers, HCNH⁺, was formed using the ion producing methods used. The reactivity of several ions C₃HnN⁺ (n = 1 → 4) and C₃HnO⁺ (n = 0 → 3), with various neutrals, was investigated to ascertain the importance of these ions in the interstellar synthesis of acrylonitrile, tricarbon monoxide and propynal. Several ion-molecule reactions of CH₂CHCN were also studied to this end. The results indicate that C₃HnN⁺ (n > 0) and C₃HnO⁺ (n > 0) are unreactive with the most prominent cloud constituents H₂ and CO; thus dissociative recombination of these ions should represent a significant source of the target molecules. Several ion-molecule reactions of the types X⁺ + CH₂HCN, and C₃nN⁺ + X, produce ions which, on dissociative recombination, are expected to yield cyanopolyynes and cyclopropenylidene. Several reactions of the C₃HnO+ ions suggest pathways to higher-order polycarbon monoxides and dioxides. The reactivity of the molecular ions of C₂N₂, C₄N₂ and C₃0₂ have also been studied, to gauge the likely consequences of reactions of such ions within interstellar clouds. The thermochemistry of the reaction HCN⁺ + CF4₄ → CF₃⁺ + HF + CN is explored with regard to the proposal that this reaction may be 'entropy-driven'. The interstellar significance of a novel class of neutral-neutral reactions has been considered. The reactivity of the ions C₄Hn⁺ (n = 0 → 4), C₃HnN⁺ (n = 0 → 4), and C₃HnO⁺ (n = 0 → 3) with the neutrals H₂, CO, C₂H₂ and HCN is discussed in greater detail. Previous studies have determined that ions featuring linear carbon-chain skeletons are more reactive with H₂ and with CO if they feature 'bare' (non-hydrogenated) terminal carbon atoms: the present study suggests that ions with bare terminal C atoms are also more reactive than ions where the terminal atom is N or 0 rather than C. This observation may be explained by the degree of carbene character evident in such ions. These results are also discussed with reference to the predominance of very highly unsaturated linear molecules within interstellar clouds.
机译:报道了与星际云化学潜在相关的几种离子分子反应系统的研究结果。使用在室温(300±5 K)下运行的选择离子流量管和压力为0.30±0.01托的氦缓冲气体进行测量。 C 1 H 2和C 2 N 2的质子亲合力通过测量涉及相似质子亲和性化合物的正向和反向质子转移反应的速率系数来确定。所得结果为PA(C 2 N 2)= 674±4kJ mol -1和PA(C 2 H 2)= 741±4kJ mol-1:根据较早的从ICR包围中获得的测量结果,后一数量明显低于文献值。研究了离子C 2 N 4,C 3 N 4,CHN 3和CH 2 N 3的异构性,利用与各种中性的反应性来区分异构体。离子CCN 3 / CNC 4和CCCN 3 / c-C 3 N 5是根据它们与H 2的反应性而区分的:在两种情况下,具有末端N原子的异构体迅速反应,而另一种异构体不反应。异构体HCN⁺/HNC⁺的鉴定由于HCN⁺向更稳定的异构体HNC⁺的互变异构的发生而变得复杂,该机制是由几种中性试剂发生的“向前”和“向后”质子转移机理:随后使用与CF₄反应区分这些异构体,因为HCN⁺与CF₄快速反应,而HNC⁺不反应。所有这些异构系​​统的反应都通过星际云中大量的中性分子进行了检查。不能根据与所调查的中性物的反应性来区分离子HCNH 3和CNH 2:我们不能排除使用所使用的离子产生方法仅形成这些异构体之一HCNH 3的可能性。研究了几种离子C₃HnN⁺(n = 1→4)和C₃HnO⁺(n = 0→3)与各种中性离子的反应性,以确定这些离子在星际合成丙烯腈,一氧化碳和丙醛中的重要性。为此,还研究了CH 2 CHCN的几种离子分子反应。结果表明,C₃HnN⁺(n> 0)和C₃HnO⁺(n> 0)与最突出的云成分H 2和CO没有反应。因此,这些离子的解离重组应代表靶分子的重要来源。 X 3 + CH 2 HCN和C 3 nN 4 + X类型的几种离子分子反应产生的离子在离解重组后可产生氰基多炔和环丙烯基。 C₃HnO+离子的一些反应提示了通往更高阶一氧化碳和二氧化碳的途径。还研究了C 2 N 2,C 3 N 2和C 0 0 2的分子离子的反应性,以评估这些离子在星际云中反应的可能结果。关于反应HCN + CF 4 + CF 4 + HF + CN的热化学,有人提出该反应可能是“熵驱动的”。已经考虑了新型中性-中性反应的星际意义。离子C₄Hn⁺(n = 0→4),C₃HnN⁺(n = 0→4)和C₃HnOn(n = 0→3)与中性H 2,CO,C 2 H 2和HCN的反应性进行了更详细的讨论。 。先前的研究已经确定,具有线性碳链骨架的离子如果具有“裸露”(未氢化)末端碳原子,则它们与H 2和CO的反应性更高:本研究表明,具有裸露的末端C原子的离子也具有更高的反应性。末端原子为N或0而不是C的离子。这种观察可以用这种离子中明显的卡宾特征度来解释。还参考星际云中高度不饱和的线性分子的优势讨论了这些结果。

著录项

  • 作者

    Petrie Simon Antony Hudson;

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  • 年度 1991
  • 总页数
  • 原文格式 PDF
  • 正文语种 en
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