Chemical constraints on organic cations in the interstellar medium
Chemical constraints on organic cations in the interstellar medium
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DOI：
10.1021/ja971330v
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发表时间：
1997-09-03
影响因子：
15
通讯作者：
Snow, TP
中科院分区：
文献类型：
--
作者：
LePage, V;Keheyan, Y;Snow, TP
70多年前首次发现的弥漫星际带是在整个星际介质中观察到的可见吸收特征;这些带的起源仍然未知。1电离的多环芳烃（PAHs）被认为是这些波段的潜在载体2 -4以及星际云的重要组成部分。脱氢多环芳烃衍生物在电离场较强的星际环境中也可能很重要。5然而，对这些PAH离子的化学反应性的研究很少6 -8，特别是关于它们在星际介质中的稳定性，除了分子氢之外，星际介质中的主要物质是原子; 9离子-原子反应的定量研究很困难。在本文中，我们报告的原型PAH阳离子C10 H8+，C10 H7+，和C10 H6+与主要的星际中性物种，包括H2和H，N，O原子的反应的研究。闭壳层C10 H7+离子与原子相对不反应，但容易与H2和其他分子缔合。[10]相反，自由基离子（C10 H8+和C10 H6+）与分子试剂不反应，但与原子试剂表现出新的反应。这些研究表明，虽然以前的光谱研究主要集中在母体自由基阳离子上，但质子化离子可能是星际介质中PAH阳离子的主要形式。实验是在串联流动的余辉选择离子流管（FA-SIFT）14中进行的，该管非常适合研究离子-原子反应。C10 H8+离子是通过萘与冷阴极放电中产生的亚稳态氩原子的潘宁电离形成的; 15这种相对温和的电离过程16干净地产生萘的母阳离子。脱氢离子C10 H7+和C10 H6+是通过萘与电子碰撞电离产生的氦离子的化学电离形成的。动力学研究表明，这些离子以环状和非环状形式存在。17反应物离子从源区域提取，质量选择，并注入反应流管，在反应流管中它们通过与氦的碰撞而热化。分子反应物通过多个入口加入，动力学作为反应距离的函数进行测量。原子氢是通过使超高纯度的H2通过微波放电管产生的;校准反应用于测量氢原子密度18-20，并表明解离率约为30%。通过使N2通过微波放电形成原子氮，并且通过用一氧化氮滴定原子氮形成原子氧：21，22 N+ NO f N2+ O。N2的解离率约为2%。对速率常数进行了小的修正，以考虑原子试剂的壁复合和混合。23还使用双室、3-T傅立叶变换离子回旋共振仪（Extrel FTMS 2001）研究了多环芳烃阳离子与分子试剂的反应。24 C10 H7+是通过电子撞击溴代萘和SiF 3+与氟代萘的置换反应生成的。25反应速率常数和产物分布汇总于表1中。虽然C10 H8+和C10 H6+不与分子氢反应，但C10 H7+容易通过缔合反应。与H2的反应生成质子化萘，在0.5 Torr的氦气压力下速率常数为5.2× 10-11 cm 3 s-1：
The diffuse interstellar bands, first detected more than 70 years ago, are visible absorption features observed throughout the interstellar medium; the origin of these bands remains unknown. 1 The ionized polycyclic aromatic hydrocarbons (PAHs) have been implicated as potential carriers of these bands2-4 as well as important components of interstellar clouds. Dehydrogenated PAH derivatives may also be important in interstellar environments where ionization fields are strong. 5 Nevertheless, there have been few studies6-8 of the chemical reactivity of these PAH ions especially regarding their stability in the interstellar medium where, with the exception of molecular hydrogen, the dominant species are atomic; 9 quantitative studies of ion-atom reactions are difficult. In this paper we report studies of the reactions of the prototypical PAH cations C10H8+, C10H7+, and C10H6+ with the major interstellar neutral species, including H2 and H, N, and O atoms. The closed shell C10H7+ ion is relatively unreactive with atoms but readily associates with H2 and other molecules. 10 In contrast, the radical ions (C10H8+ and C10H6+) are unreactive with molecular reagents, but exhibit novel reactions with the atomic reagents. These studies suggest that, although previous spectroscopic studies have focused on the parent radical cations, 11-13 the protonated ions are likely to be the dominant forms of PAH cations in the interstellar medium. The experiments were carried out in a tandem flowing afterglow-selected ion flow tube (FA-SIFT), 14 which is ideally suited to the study of ion-atom reactions. The C10H8+ ion is formed by Penning ionization of naphthalene with metastable argon atoms which are generated in a cold cathode discharge; 15 this relatively gentle ionization process16 cleanly generates the parent cation of naphthalene. The dehydrogenated ions, C10H7+ and C10H6+, are formed by chemical ionization of naphthalene with helium ions that are produced by electron impact ionization. Kinetic studies demonstrate that these ions exist in both cyclic and acyclic forms. 17 The reactant ions are extracted from the source region, mass-selected, and injected into the reaction flow tube where they are thermalized by collisions with helium. Molecular reactants are added through a manifold of inlets, and kinetics are measured as a function of reaction distance. Atomic hydrogen is generated by passing ultra-high-purity H2 through a microwave discharge tube; calibration reactions are used to measure the hydrogen atom density, 18-20 and indicate that dissociation ratios are about 30%. Atomic nitrogen is formed by passing N2 through the microwave discharge, and atomic oxygen is formed by titration of atomic nitrogen with nitric oxide: 21, 22 N+ NO f N2+ O. Dissociation ratios of N2 are about 2%. Small corrections to the rate constants are made to account for wall recombination and mixing of the atomic reagents. 23 Studies of the reactions of PAH cations with molecular reagents were also carried out with a dual-cell, 3-T, Fourier transform-ion cyclotron resonance instrument (Extrel FTMS 2001). 24 C10H7+ was formed both by electron impact on bromonaphthalene and by a displacement reaction of SiF3+ with fluoronaphthalene. 25The reaction rate constants and product distributions are summarized in Table 1. While C10H8+ and C10H6+ are unreactive with the molecular hydrogen, C10H7+ reacts readily by association. The reaction with H2 generates protonated naphthalene with a rate constant of 5.2× 10-11 cm3 s-1 at 0.5 Torr of helium pressure: