Gerlich D
Faculty of Natural Science, University of Technology, Chemnitz 09107, Germany.
Phys Chem Chem Phys. 2005 Apr 7;7(7):1583-91. doi: 10.1039/b419328h.
Numerous recent calculations have provided a rather detailed picture how the protonated methane, CHS+, really may look like at very low temperatures; however, there is not yet any experiment, providing information on the correlation of a structure of this fluxional ion with a state to state transition induced by a photon or a collision. Various efforts in spectroscopy and mass spectrometry have contributed important pieces to the puzzle but there are no real final conclusions, e.g. infrared spectra in the region of the C-H stretching vibration are waiting for assignment since several years. This contribution reviews and discusses the potential and the limitations of a variety of detailed collision experiments for learning more about protonated methane and deuterated variants, either via creation, modification or destruction of CH5+. There has been a controversial discussion about an experiment which seemed to indicate that deuteron transfer from CD4+ to CH4 can create stable isotopomers with chemically distinguishable hydrogen atoms, CH3-HD+. Detailed integral and differential cross sections measured with sophisticated ion beam techniques revealed interesting dynamics; but, unfortunately, CH5+ formation is certainly more complicated than just a simple proton transfer into a stable isomer. In collisions of CH4+ with CH4 or CD4, there is significant scrambling and one would need to use differential scattering selection for getting ions produced exclusively via a specific mechanism. There have been several low temperature ion trap studies leading to CH5+, e.g., simply via radiative association of CH3+ with H2 or via hydrogen abstraction in CH4(+) + H2 collisions. Very interesting and in some cases unforeseen observations have been made by using deuterated variants in low temperature collisions. A general conclusion is that H-D exchange is not only influenced by the differences in zero point energies but that symmetry selection rules can significantly restrict the scrambling. For example, conservation of nuclear spin may allow one to synthesize specific CD3H2+ ions with a local ortho hydrogen rotator via radiative association. Cold trapped CH5+ ions have been probed by collisions with HD. Despite the high sensitivity of the trapping technique, no H-D exchange could be observed at all while a few isotopic equivalents of CH7+ grow via radiative association. Finally, our most recent activities are briefly mentioned, probing cold CHR+ ions via collisions with slow H or D atoms. This survey ends with a conclusion and an outlook. It is very sure that all traditional methods of collisional probing are unable to provide evidence for different CH5+ isomers. If, however, spectroscopy, low temperature collision dynamics, multi-electrode traps and supersonic or effusive beams are combined in a suitable way, one may succeed in probing single states of unperturbed cold ions at low temperatures.
最近的大量计算已经相当详细地描绘了在极低温度下质子化甲烷（CH₅⁺）的真实模样；然而，尚未有任何实验能提供关于这种易变离子的结构与由光子或碰撞引起的态到态跃迁之间相关性的信息。光谱学和质谱学方面的各种努力为这一难题拼凑出了重要的部分，但尚未得出真正的最终结论，例如，C - H伸缩振动区域的红外光谱已经多年等待归属。本论文综述并讨论了各种详细碰撞实验的潜力和局限性，这些实验旨在通过CH₅⁺的产生、改性或破坏来更多地了解质子化甲烷及其氘代变体。关于一个实验存在争议性的讨论，该实验似乎表明从CD₄⁺到CH₄的氘转移可以产生具有化学可区分氢原子的稳定同位素异构体CH₃ - HD⁺。用复杂的离子束技术测量的详细积分和微分截面揭示了有趣的动力学；但不幸的是，CH₅⁺的形成肯定比简单的质子转移到稳定异构体要复杂得多。在CH₄⁺与CH₄或CD₄的碰撞中，存在显著的重排，需要使用微分散射选择来获得仅通过特定机制产生的离子。已经有几项低温离子阱研究产生了CH₅⁺，例如，简单地通过CH₃⁺与H₂的辐射缔合或通过CH₄(⁺) + H₂碰撞中的氢提取。通过在低温碰撞中使用氘代变体，已经取得了非常有趣且在某些情况下是意想不到的观测结果。一个普遍的结论是，H - D交换不仅受到零点能量差异的影响，而且对称选择规则可以显著限制重排。例如，核自旋守恒可能允许通过辐射缔合合成具有局部邻位氢转子体的特定CD₃H₂⁺离子。通过与HD的碰撞探测了冷捕获的CH₅⁺离子。尽管捕获技术具有高灵敏度，但根本没有观察到H - D交换，而一些CH₇⁺的同位素等效物通过辐射缔合增加。最后，简要提及了我们最近的活动，即通过与慢H或D原子的碰撞探测冷CHR⁺离子。本综述以结论和展望结束。可以肯定的是，所有传统的碰撞探测方法都无法为不同的CH₅⁺异构体提供证据。然而，如果以合适的方式将光谱学、低温碰撞动力学、多电极阱以及超音速或分子束相结合，人们可能成功地在低温下探测未受干扰的冷离子的单个态。
