Yang Shengfu, Brereton Scott M, Wheeler Martyn D, Ellis Andrew M
Department of Chemistry, University of Leicester, University Road, Leicester, UKLE1 7RH.
Phys Chem Chem Phys. 2005 Dec 21;7(24):4082-8. doi: 10.1039/b511628g. Epub 2005 Oct 10.
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.
记录了一系列包裹在大型超流液氦纳米液滴（约60,000个氦原子）中的C1 - C6醇的电子轰击（70 eV）质谱。与气相相比，氦的存在改变了碎裂模式，一些离子产物通道比其他通道受到的影响更大，最显著的是母离子中C(α)-H键的断裂以形成相应的氧鎓离子。氦也增强了母离子强度，但仅对于所研究的两种环醇，环戊醇和环己醇，这种效应大到足以将母离子从次要产物（在气相中）转变为氦滴实验中最丰富的离子。为了证明这些发现并非醇类所独有，我们还研究了几种醚。对于醇类和醚类所获得的结果很难仅通过周围超流氦对激发母离子的快速冷却来解释，尽管这无疑会发生。似乎还涉及第二个因素，即一种笼效应，它比其他碎裂通道更有利于氢原子损失。这项工作中所研究的分子表明，掺杂氦纳米液滴的电子轰击电离不会产生足够大的软化效应，从而无法用于分析质谱。
