Liu Chongming, Nishshanka Upul, Attygalle Athula B
Center for Mass Spectrometry, Department of Chemistry, Chemical Biology, and Biomedical Engineering, Stevens Institute of Technology, Hoboken, NJ, 07030, USA.
J Am Soc Mass Spectrom. 2016 May;27(5):927-39. doi: 10.1007/s13361-016-1357-5. Epub 2016 Feb 25.
The anilide anion (m/z 92) generated directly from aniline, or indirectly as a fragmentation product of deprotonated acetanilide, captures CO2 readily to form the carbamate anion (m/z 136) in the collision cell, when CO2 is used as the collision gas in a tandem-quadrupole mass spectrometer. The gas-phase affinity of the anilide ion to CO2 is significantly higher than that of the phenoxide anion (m/z 93), which adds to CO2 only very sluggishly. Our results suggest that the efficacy of CO2 capture depends on the natural charge density on the nitrogen atom, and relative nucleophilicity of the anilide anion. Generally, conjugate bases generated from aniline derivatives with proton affinities (PA) less than 350 kcal/mol do not tend to add CO2 to form gaseous carbamate ions. For example, the anion generated from p-methoxyaniline (PA = 367 kcal/mol) reacts significantly faster than that obtained from p-nitroaniline (PA = 343 kcal/mol). Although deprotonated p-aminobenzoic acid adds very poorly because the negative charge is now located primarily on the carboxylate group, it reacts more efficiently with CO2 if the carboxyl group is esterified. Moreover, mixture of CO2 and He as the collision gas was found to afford more efficient adduct formation than CO2 alone, or as mixtures made with nitrogen or argon, because helium acts as an effective "cooling" gas and reduces the internal energy of reactant ions.
当在串联四极杆质谱仪中使用二氧化碳作为碰撞气体时，直接由苯胺生成的酰苯胺阴离子（m/z 92），或作为去质子化乙酰苯胺的碎片产物间接生成的酰苯胺阴离子，在碰撞池中很容易捕获二氧化碳，形成氨基甲酸酯阴离子（m/z 136）。酰苯胺离子对二氧化碳的气相亲和力明显高于苯氧阴离子（m/z 93），后者与二氧化碳的反应非常缓慢。我们的结果表明，二氧化碳捕获的效率取决于氮原子上的自然电荷密度以及酰苯胺阴离子的相对亲核性。一般来说，由质子亲和力（PA）小于350 kcal/mol的苯胺衍生物生成的共轭碱不易添加二氧化碳形成气态氨基甲酸酯离子。例如，对甲氧基苯胺（PA = 367 kcal/mol）生成的阴离子反应速度明显快于对硝基苯胺（PA = 343 kcal/mol）生成的阴离子。虽然去质子化的对氨基苯甲酸反应很差，因为负电荷现在主要位于羧基上，但如果羧基被酯化，它与二氧化碳的反应会更有效。此外，发现二氧化碳和氦气的混合气体作为碰撞气体比单独的二氧化碳或与氮气或氩气混合时能提供更有效的加合物形成，因为氦气作为一种有效的“冷却”气体，降低了反应物离子的内能。
