Organic free radicals in superheated water studied by muon spin spectroscopy.
Organic free radicals in superheated water studied by muon spin spectroscopy.
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DOI：
10.1021/ja0537070
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发表时间：
2005-09
影响因子：
15
通讯作者：
P. W. Percival;J. Brodovitch;K. Ghandi;Brett M. McCollum;I. McKenzie
中科院分区：
文献类型：
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作者：
P. W. Percival;J. Brodovitch;K. Ghandi;Brett M. McCollum;I. McKenzie
人们迫切需要在高温高压下识别和监测水中的反应中间体，但传统技术在这种具有挑战性的条件下研究瞬时自由基的能力有限。现在已经开发了一种装置，允许在过热的水中进行有机自由基的µ子避免能级交叉光谱(MuLCR)。MuLCR和横场Muon自旋旋转(Tf-muSR)的结合提供了通过核超精细耦合常数来识别和表征自由基的手段。由于这些自由基是由不饱和化合物中加成的Muon(Mu=Mu+e-)产生的，所以随后的Muonated自由基对应于传统的有机自由基，但用一个Muon自旋标记取代了其中一个质子。Muon自旋光谱是目前唯一一种在水热条件下不受其他反应中间体干扰的、以明确方式表征瞬时自由基的技术。本文从醇的脱水反应和酮的烯醇化反应两个方面举例说明了如何利用离子化自由基来监测水热体系中存在的物种。在过热的水中(通常为350摄氏度，250巴)，显示了以下自由基的突变形式的光谱并报告了超精细常数：2-丙基、2-甲基-2-丙基(叔丁基)和2-羟基-2-丙基。后者是丙酮的酮和烯醇两种形式的异构体加成的产物，但根据反应通道的不同会产生不同的同位异构体。这在未来对烯醇在燃烧中的作用的研究中应该被证明是非常有价值的。
There is a pressing need to identify and monitor reaction intermediates in water at high temperatures and pressures, but conventional techniques have limited capability for studying transient free radicals under such challenging conditions. Apparatus has now been developed to permit muon avoided-level crossing spectroscopy (muLCR) of organic free radicals in superheated water. The combination of muLCR with transverse-field muon spin rotation (TF-muSR) provides the means to identify and characterize free radicals via their nuclear hyperfine coupling constants. Because the radicals are derived from the addition of muonium (Mu = mu+ e-) to unsaturated compounds, the ensuing muoniated free radicals correspond to conventional organic free radicals but with a muon spin label substituted for one of the protons. Muon spin spectroscopy is the only technique presently being used to characterize transient free radicals under hydrothermal conditions in an unambiguous manner, free from interference from other reaction intermediates. This paper demonstrates how muoniated radicals can be used to monitor the species present in hydrothermal systems, and examples are presented from two classes of reaction: dehydration of alcohols and enolization of ketones. Spectra are displayed and hyperfine constants reported for muoniated forms of the following free radicals in superheated water (typically 350 degrees C at 250 bar): 2-propyl, 2-methyl-2-propyl (tert-butyl), and 2-hydroxy-2-propyl. The latter radical is the product of muonium addition to both the keto and the enol forms of acetone, but different isotopomers are produced according to which reaction channel is dominant. This should prove invaluable in future studies of the role of enols in combustion.