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首页> 外文期刊>Journal of mass spectrometry: JMS >Fragmentation of oxime and silyl oxime ether odd-electron positive ions by the McLafferty rearrangement: new insights on structural factors that promote, β fragmentation
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Fragmentation of oxime and silyl oxime ether odd-electron positive ions by the McLafferty rearrangement: new insights on structural factors that promote, β fragmentation

机译：麦克拉菲重排使肟和甲硅烷基肟醚奇电子正离子断裂：促进β断裂的结构因子的新见解

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摘要

The McLafferty rearrangement is an extensively studied fragmentation reaction for the odd-electron positive ions from a diverse range of functional groups and molecules. Here, we present experimental and theoretical results of 12 model compounds that were synthesized and investigated by GC-TOF MS and density functional theory calculations. These compounds consisted of three main groups: carbonyls, oximes and silyl oxime ethers. In all electron ionization mass spectra, the fragment ions that could be attributed to the occurrence of a McLafferty rearrangement were observed. For t-butyldimethylsilyl oxime ethers with oxygen in a β-position, the McLafferty rearrangement was accompanied by loss of the t-butyl radical. The various mass spectra showed that the McLafferty rearrangement is relatively enhanced compared with other primary fragmentation reactions by the following factors: oxime versus carbonyl, oxygen versus methylene at the β-position and ketone versus aldehyde. Calculations predict that the stepwise mechanism is favored over the concerted mechanism for all but one compound. For carbonyl compounds, C-C bond breaking was the rate-determining step. However, for both the oximes and t-butyldimethylsilyl oxime ethers with oxygen at the β-position, the hydrogen transfer step was rate limiting, whereas with a CH_2 group at the β-position, the C-C bond breaking was again rate determining. n-Propoxy-acetaldehyde, bearing an oxygen atom at the β-position, is the only case that was predicted to proceed through a concerted mechanism. The synthesized oximes exist as both the (E)-and (Z)-isomers, and these were separable by GC. In the mass spectra of the two isomers, fragment ions that were generated by the McLafferty rearrangement were observed. Finally, fragment ions corresponding to the McLafferty reverse charge rearrangement were observed for all compounds at varying relative ion intensities compared with the conventional McLafferty rearrangement.
机译：McLafferty重排是对来自各种官能团和分子的奇数电子正离子进行的广泛研究的裂解反应。在这里，我们介绍了通过GC-TOF MS和密度泛函理论计算合成和研究的12种模型化合物的实验和理论结果。这些化合物由三个主要组组成：羰基，肟和甲硅烷基肟醚。在所有电子电离质谱图中，均观察到可能归因于麦克拉菲重排的碎片离子。对于氧在β-位的叔丁基二甲基甲硅烷基肟醚，McLafferty重排伴随着叔丁基自由基的损失。各种质谱表明，与其他主要的片段化反应相比，麦克拉菲重排相对受到以下因素的影响相对较大：肟相对于羰基，氧相对于亚甲基的β位以及酮相对于醛。计算结果表明，除一种化合物外，逐步机理优于协调机理。对于羰基化合物，C-C键断裂是决定速率的步骤。然而，对于在β位具有氧的肟和叔丁基二甲基甲硅烷基肟醚，氢转移步骤是速率限制的，而在β位具有CH_2基团的情况下，C-C键的断裂再次决定了速率。在β-位带有氧原子的正丙氧基乙醛是唯一预计会通过协同机制进行的情况。合成的肟以（E）和（Z）异构体形式存在，并且可以通过GC分离。在两种异构体的质谱图中，观察到由麦克拉菲重排产生的碎片离子。最后，与常规的麦克拉菲重排相比，在变化的相对离子强度下，所有化合物均观察到了与麦克拉菲逆电荷重排相对应的碎片离子。

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