Pure Samples of Individual Conformers: The Separation of Stereoisomers of Complex Molecules Using Electric Fields
Pure Samples of Individual Conformers: The Separation of Stereoisomers of Complex Molecules Using Electric Fields
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DOI：
10.1002/anie.200902650
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发表时间：
2009-01-01
影响因子：
16.6
通讯作者：
Stapelfeldt, Henrik
中科院分区：
文献类型：
--
作者：
Filsinger, Frank;Kuepper, Jochen;Stapelfeldt, Henrik
许多复杂分子具有多个结构异构体；即在它们的势能面上存在多个局部极小值。大约25年前，人们观察到色氨酸的多种构象即使在超音速喷气机的几开尔文的低温下也会存在从那时起，这些构象已经用复杂的光谱技术进行了广泛的研究。单个构象可以从其不同的电子[1,2]或微波[3]光谱中识别出来。例如，可以使用微波[4]或多重共振红外光谱来获得构象结构的信息。[5,6]在类似的实验中，甚至可以获得分离构象屏障的信息空间分离构象的制备将为进一步研究提供独特的可能性。反应散射实验可以直接研究不同物质的化学性质及其差异。这样的纯样品也将使一类新的实验成为可能，例如电子[8]和x射线衍射[9,10]或气相复杂分子的层析成像[11]实验。分子框架光电子角分布、超快时间分辨光电子能谱和超快动力学研究[12]也将受益于这些纯样品的可用性。对于带电物质，不同形状的分子的分离已被证明利用离子迁移在漂移管。[13,14]对于中性分子，分子束中构象的丰富度可以部分地受到膨胀早期选择性过势垒激发或改变载气的影响在这里，我们证明了静电偏转，一种可以追溯到20世纪20年代的经典分子束操纵方法[17,18]，当它应用于由最先进的脉冲超音速膨胀源产生的旋转冷分子的强束时，允许中性分子的构象的空间分离利用静电偏转来分离量子态的想法已经由斯特恩在1926年构想出来，用于轻双原子分子[20]，这些想法最近被扩展到大分子构象分离的建议中。(21、22)
Many complex molecules have multiple structural isomers; that is, multiple local minima on their potential energy surface. About twenty-five years ago, it was observed that multiple conformers of tryptophan are present even at the low temperatures of a few Kelvin in a supersonic jet.[1] These conformers have been studied extensively since then with sophisticated spectroscopic techniques. Individual conformers can be identified from their distinct electronic [1, 2] or microwave [3] spectra. Information on the conformational structures can be obtained using microwave [4] or multipleresonance infrared spectroscopy, for example.[5, 6] In similar experiments it was even possible to obtain information on the barriers separating the conformers.[7] The preparation of spatially separated conformers would provide unique possibilities for advanced further investigations. The chemical properties of the individual species and their differences could be directly studied in reactive scattering experiments. Such pure samples would also enable a new class of experiments, such as electron [8] and X-ray diffraction [9, 10] or tomographic imaging [11] experiments of complex molecules in the gas phase. Molecular-frame photoelectron angular distributions, ultrafast time-resolved photoelectron spectroscopy, and ultrafast dynamics studies [12] would also benefit from the availability of these pure samples. For charged species, the separation of molecules with different shapes has been demonstrated by utilizing ion mobility in drift tubes.[13, 14] For neutral molecules, the abundance of the conformers in molecular beams can be partly influenced by selective over-the-barrier excitation in the early stage of the expansion [15] or by changing the carrier gas.[16] Herein, we demonstrate that electrostatic deflection, a classic molecular beam manipulation method that dates back to the 1920s,[17, 18] allows the spatial separation of the conformers of a neutral molecule when it is applied to intense beams of rotationally cold molecules produced by a state-ofthe-art pulsed supersonic expansion source.[19] The idea of exploiting electrostatic deflection to separate quantum states was already conceived by Stern in 1926 for light diatomic molecules,[20] and these ideas were recently extended to proposals for the separation of conformers of large molecules.[21, 22]