Competitive Deprotonation and Superoxide [O2-•] Radical-Anion Adduct Formation Reactions of Carboxamides under Negative-Ion Atmospheric-Pressure Helium-Plasma Ionization (HePI) Conditions
Competitive Deprotonation and Superoxide [O2-•] Radical-Anion Adduct Formation Reactions of Carboxamides under Negative-Ion Atmospheric-Pressure Helium-Plasma Ionization (HePI) Conditions
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DOI：
10.1007/s13361-015-1296-6
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发表时间：
2016-03-01
影响因子：
3.2
通讯作者：
Attygalle, Athula B.
中科院分区：
文献类型：
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作者：
Hassan, Isra;Pinto, Spencer;Attygalle, Athula B.
已知带有N-H官能团的羧酰胺在负离子产生质谱条件下经历去质子化。在此，我们报告，N-H轴承羧酰胺的酸度低于氢过氧自由基（HO-O-中心点）优先形成超氧自由基-阴离子（O-2（-中心点））加合物，而不是去质子化，当它们暴露于辉光放电的氦等离子体电离源。例如，N-烷基乙酰胺的光谱显示超氧自由基-阴离子（O-2（-中心点））加合物的峰。相反，酸性更强的酰胺，如N-烷基三氟乙酰胺，在类似的实验条件下优先进行去质子化。N-烷基乙酰胺的O-2（-中心点）加合物在碰撞活化时失去中性酰胺或过氧化氢自由基（HO-O-中心点），分别生成超氧阴离子自由基（m/z 32）或去质子化酰胺[m/z（M-H）（-）]。对于稍微酸性的羧酰胺，两个实体之间的关联是弱的。因此，在最温和的碰撞活化时，加合物解离以排出超氧阴离子。超氧化物加合物形成的结果是有用的结构测定的目的，因为缺乏N-H功能的羧酰胺进行既不去质子化，也不加合物的形成下的HePI条件。
Carboxamides bearing an N-H functionality are known to undergo deprotonation under negative-ion-generating mass spectrometric conditions. Herein, we report that N-H bearing carboxamides with acidities lower than that of the hydroperoxyl radical (HO-O-center dot) preferentially form superoxide radical-anion (O-2 (-center dot)) adducts, rather than deprotonate, when they are exposed to the glow discharge of a helium-plasma ionization source. For example, the spectra of N-alkylacetamides show peaks for superoxide radical-anion (O-2 (-center dot)) adducts. Conversely, more acidic amides, such as N-alkyltrifluoroacetamides, preferentially undergo deprotonation under similar experimental conditions. Upon collisional activation, the O-2 (-center dot) adducts of N-alkylacetamides either lose the neutral amide or the hydroperoxyl radical (HO-O-center dot) to generate the superoxide radical-anion (m/z 32) or the deprotonated amide [m/z (M - H)(-)], respectively. For somewhat acidic carboxamides, the association between the two entities is weak. Thus, upon mildest collisional activation, the adduct dissociates to eject the superoxide anion. Superoxide-adduct formation results are useful for structure determination purposes because carboxamides devoid of a N-H functionality undergo neither deprotonation nor adduct formation under HePI conditions.