Soft or hard ionization of molecules in helium nanodroplets? An electron impact investigation of alcohols and ethers.
Soft or hard ionization of molecules in helium nanodroplets? An electron impact investigation of alcohols and ethers.
复制标题
氦纳米液滴中分子的软电离还是硬电离？
DOI：
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发表时间：
2005
期刊：
影响因子：
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通讯作者：
A. Ellis
中科院分区：
文献类型：
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作者：
Shengfu Yang;Scott M. Brereton;Martyn D. Wheeler;A. Ellis
记录了一系列被大的超流液氦纳米液滴(约60,000个氦原子)包裹的C_1-C_6醇的电子碰撞(70 EV)质谱图。与气相相比，氦的存在改变了碎裂模式，一些离子产物通道受到的影响比其他通道更大，最显著的是母离子中C(α)-H键的断裂形成相应的氧离子。氦也增强了母体离子的强度，但只有对于所研究的两种环戊醇和环己醇，这种影响才足够大，足以在氦滴实验中将母体离子从次要产物(气相中)转变为最丰富的离子。为了证明这些发现并不是酒精独有的，我们还研究了几种醚。对于醇和醚所得到的结果，很难仅仅通过激发的母离子被周围的超流氦快速冷却来解释，尽管这无疑是发生的。第二个因素似乎也涉及到，笼子效应，它比其他碎片通道更有利于氢原子的损失。这项工作中探索的一组分子表明，掺杂的氦纳米液滴的电子碰撞电离并不能提供足够大的软化效应来用于分析质谱学。
Electron impact (70 eV) mass spectra of a series of C1-C6 alcohols encased in large superfluid liquid helium nanodroplets (approximately 60,000 helium atoms) have been recorded. The presence of helium alters the fragmentation patterns when compared with the gas phase, with some ion product channels being more strongly affected than others, most notably cleavage of the C(alpha)-H bond in the parent ion to form the corresponding oxonium ion. Parent ion intensities are also enhanced by the helium, but only for the two cyclic alcohols studied, cyclopentanol and cyclohexanol, is this effect large enough to transform the parent ion from a minor product (in the gas phase) into the most abundant ion in the helium droplet experiments. To demonstrate that these findings are not unique to alcohols, we have also investigated several ethers. The results obtained for both alcohols and ethers are difficult to explain solely by rapid cooling of the excited parent ions by the surrounding superfluid helium, although this undoubtedly takes place. A second factor also seems to be involved, a cage effect which favors hydrogen atom loss over other fragmentation channels. The set of molecules explored in this work suggest that electron impact ionization of doped helium nanodroplets does not provide a sufficiently large softening effect to be useful in analytical mass spectrometry.