Ion-molecule reactions in helium nanodroplets doped with C60 and water clusters.
Ion-molecule reactions in helium nanodroplets doped with C60 and water clusters.
复制标题
掺杂 C60 和水簇的氦纳米液滴中的离子分子反应。
DOI：
10.1002/anie.200904381
复制
发表时间：
2009
影响因子：
--
通讯作者：
P. Scheier
中科院分区：
文献类型：
--
作者：
S. Denifl;F. Zappa;I. Mähr;F. Ferreira da Silva;A. Aleem;A. Mauracher;M. Probst;J. Urban;P. Mach;A. Bacher;O. Echt;T. Märk;P. Scheier
氦纳米液滴含有大约10到10个原子，为重要的新实验提供了一条途径。在低压环境中，液滴通过弱结合氦原子的蒸发在微秒内冷却到0.37 K。在与超流液滴碰撞中捕获的分子将在液滴内部迅速聚集成新颖的，通常是亚稳态的结构。这些“个人纳米恒温器”可用于探索化学反应。例如，M ller等人报道电离引发液滴内铯-水络合物的完全水解。本文报道了C60与小分子水团簇之间的离子-分子反应。水是生物分子组织的一个组成部分;其生物活性可以通过表征其在C60-H2O界面的功能来进一步理解。[4]C60是疏水性的;其0.5 nm的硬核半径接近交叉点，超过该交叉点氢键的断裂变得不可避免。分子动力学模拟表明，富勒烯与单链和双链DNA强烈结合;在饮用水中加入水合C60可以减轻乙醇对大鼠脑细胞的损伤，而不会引起任何不良的生物学效应。在本工作中，氦液滴掺杂C60和水，随后通过电子碰撞电离。实验结果的解释是辅助从头算Hartree-Fock计算。两个观察站出来：第一，中性C60和水之间的弱相互作用扩展到阳离子系统。对于某些水簇尺寸，发生整个水簇的解吸而不是水分子的蒸发损失。其次，C60 OH+是主要的产物离子。我们假设，这个离子的结果从双电荷[C60（H2O）] 2+中间体，形成从初级He离子的电荷转移。掺杂氦液滴中双电荷中间体的存在及其在随后的离子-分子反应中的作用迄今为止一直被忽视;这些中间体为以前观察到的有机分子和生物分子簇的氢损失提供了令人信服的理由。我们首先总结了通过离子化的氦液滴掺杂水（无论是H2O或D2 O），但没有C60。与先前的报告一致，[11]电子碰撞电离导致了一系列突出的质子化水簇离子。观察到未质子化的水簇离子相对于质子化的簇离子具有10%的丰度。未质子化的离子是不可观察的电子碰撞或裸水集群的多光子电离，但它们发生，如果水集群与重稀有气体原子络合。这些趋势是很好理解的（H2O）2 +的基态对应于质子转移异构体OH-H3 O，它的解离OH + H3 O+比解离H2O + H2O更容易。水分子团簇的直接从头算动力学研究表明，垂直电离之后是一个或多个无势垒质子转移反应在100 fs内;溶剂重组导致一个高度兴奋的簇离子和喷射的OH自由基在亚皮秒;足够的能量仍然蒸发几个水分子。当氦液滴与C60共掺杂时，团簇离子的组成发生了剧烈的变化。图1中最突出的离子系列来自C60（D2 O）n，n = 0，1，2。脱氢离子，也就是说，离子的化学计量C60（D2 O）n1 OD+也被观察到，而质子化离子的丰度是弱的。为了进行定量分析，我们通过四个高斯集合拟合C60同位素体的分布，其中固定的振幅比是从[*] O.埃赫特物理系，新罕布什尔州达勒姆大学，NH 03824（美国）传真：（+ 1）603-862-2998电子邮件：olof. http://www.physics.unh.edu/ unh.edu
Helium nanodroplets, which contain some 10 to 10 atoms, provide an avenue for important new experiments. In a lowpressure environment, the droplets cool within microseconds to 0.37 K by evaporation of weakly bound helium atoms. Molecules captured in collisions with a superfluid droplet will quickly aggregate in the droplet s interior into novel, often metastable structures. These “personal nanocryostats” may be used to explore chemical reactions. For example, M ller et al. reported that ionization initiates complete hydrolysis of cesium–water complexes within the droplets. Herein we report ion–molecule reactions between C60 and small water clusters. Water is an integral part of biomolecular organization; its bioactivity can be further understood by characterization of its function at the C60–H2O interface. [4] C60 is hydrophobic; its hard-core radius of 0.5 nm is close to the crossover point beyond which the breakage of hydrogen bonds becomes unavoidable. Molecular dynamics simulations show that fullerenes strongly bind to single and doublestrand DNA; addition of hydrated C60 to drinking water has been found to mitigate damage of ethanol to brain cells of rats without causing any adverse biological effects. In the present work, helium droplets were doped with C60 and water, and subsequently ionized by electron impact ionization. The interpretation of experimental results was aided by ab initio Hartree–Fock calculations. Two observations stand out: Firstly, the weak interaction between neutral C60 and water extends to the cationic system. Desorption of entire water clusters rather than evaporative loss of water molecules occurs for certain water cluster sizes. Secondly, C60OH + is a major product ion. We postulate that this ion results from doubly charged [C60(H2O)] 2+ intermediates that form by charge transfer from a primary He ion. The existence of doubly charged intermediates in doped helium droplets and their role in subsequent ion–molecule reactions has so far been ignored; these intermediates provide a compelling rationale for previous observations of hydrogen loss from clusters of organic molecules and biomolecules. We first summarize results obtained by ionization of helium droplets doped with water (either H2O or D2O) but no C60 . In agreement with a previous report, [11] electron impact ionization results in a prominent series of protonated water cluster ions. Unprotonated water cluster ions are observed with a 10 % abundance relative to the protonated cluster ions. Unprotonated ions are not observable upon electron impact or multiphoton ionization of bare water clusters, but they occur if water clusters are complexed with heavy rare-gas atoms. These trends are well understood—the ground state of (H2O)2 + corresponds to the proton-transferred isomer OH–H3O , and its dissociation to OH + H3O + is energetically much more facile than dissociation to H2O + H2O . Direct ab initio dynamics studies of water clusters show that vertical ionization is followed by one or more barrierless proton transfer reactions within 100 fs; solvent reorganization leads to a highly excited cluster ion and ejection of the OH radical within sub-picoseconds; enough energy remains for the evaporation of several more water molecules. The composition of cluster ions changes drastically when helium droplets are co-doped with C60 . The most prominent ion series in Figure 1 arises from C60(D2O)n , n = 0, 1, 2. Dehydrogenated ions, that is, ions with the stoichiometry C60(D2O)n 1OD + are also observed, while the abundance of protonated ions is weak. For a quantitative analysis, we fitted the distribution of C60 isotopologues by sets of four Gaussians with fixed ratios of amplitudes computed from the 1.11 % natural abundance of [*] Prof. Dr. O. Echt Department of Physics, University of New Hampshire Durham, NH 03824 (USA) Fax: (+ 1)603-862-2998 E-mail: olof.echt@unh.edu Homepage: http://www.physics.unh.edu/