Institute of Ecological Chemistry, Helmholtz Center Munich, German Research Center for Environmental Health, Ingolstädter Landstr.1, D-85758 Neuherberg, Germany.
Rapid Commun Mass Spectrom. 2010 Apr 30;24(8):1198-206. doi: 10.1002/rcm.4504.
Glutaric acid, a common short-chain aliphatic dicarboxylic acid, was investigated in the negative ion mode by subjecting its M-H ion to collision-induced dissociation (CID) experiments in an infinity ion cyclotron resonance (ICR) cell coupled to a hexapole-quadrupole-hexapole ion guide. A 12 Tesla magnet was used for high-resolution measurements. Two distinctive main pathways were observed in the MS/MS spectrum. The fragmentation pathways were also thoroughly investigated in a density functional theory (DFT) study involving a B3LYP/6-311+G(2d,p)//B3LYP/6-311+G(d,p) level of theory. Elimination of CO(2) from the M-H ion of the dicarboxylic acid takes place in a concerted mechanism, by which a 1,5 proton shift occurs from the intact carboxyl group to the methylene moiety located in the alpha position relative to the deprotonated carboxyl group. This concerted mechanism stabilizes the terminal negative charge and deprotonates the second carboxylic acid group. Water elimination from the M-H ion does not take place by means of a simple proton removal from the alpha methylene group - and OH(-) release from the carboxylate group to abstract an additional alpha proton thus leading to the formation of a deprotonated ketene anion. In the case of this dicarboxylic acid, a new mechanism was found for water elimination, which differs from that known for aliphatic monocarboxylic acids. An intramolecular interaction between the deprotonated and the intact carboxyl groups plays a key role in making a new energetically favourable mechanism. The DFT study also reveals that a combined loss of CO2 and H2O in the form of H2CO3 is possible.
戊二酸是一种常见的短链脂肪族二羧酸，在无限离子回旋共振（ICR）池中，通过使M-H离子在负离子模式下进行碰撞诱导解离（CID）实验，对其进行了研究。该实验使用了一个 12 特斯拉的磁体进行高分辨率测量。在 MS/MS 谱中观察到了两种独特的主要途径。在涉及 B3LYP/6-311+G(2d,p)//B3LYP/6-311+G(d,p)理论水平的密度泛函理论（DFT）研究中，还彻底研究了这些碎片途径。二羧酸的M-H离子中通过协同机制消除 CO(2)，其中从完整的羧基到位于相对于去质子化羧基的α位的亚甲基部分发生 1,5 质子转移。这种协同机制稳定了末端负电荷并使第二个羧酸基团去质子化。M-H离子中的水消除不是通过从α亚甲基简单地去除质子-和从羧酸盐基团释放 OH(-)来抽象额外的α质子从而导致形成去质子化的烯酮阴离子来进行的。对于这种二羧酸，发现了一种与已知的脂肪族一元羧酸不同的水消除新机制。去质子化和完整羧基之间的分子内相互作用在形成新的能量有利的机制中起着关键作用。DFT 研究还表明，以 H2CO3 的形式同时失去 CO2 和 H2O 是可能的。
