Pashynska Vlada A, Kosevich Marina V, Gömöry Agnes, Szilágyi Zoltan, Vékey Karoly, Stepanian Stepan G
B. Verkin Institute for Low Temperature Physics and Engineering of the National Academy of Sciences of Ukraine, 47 Lenin Avenue, Kharkov 61103, Ukraine.
Rapid Commun Mass Spectrom. 2005;19(6):785-97. doi: 10.1002/rcm.1846.
In the course of a liquid secondary ion mass spectrometric (SIMS) investigation on a bisquaternary ammonium antimicrobial agent, decamethoxinum, unusual pathways of fragmentation of the organic dication M2+ of this bisquaternary salt, with preservation of the doubly charged state of the fragments, were observed. To reveal the structural and electronic parameters of decamethoxinum, which are responsible for the stabilization of its organic dication in the gas phase, a comprehensive SIMS study using metastable decay, collision-induced dissociation and kinetic energy release techniques complemented by ab initio quantum chemical calculations was performed. Pathways of fragmentation of two main precursors originating from decamethoxinum-organic dication M2+ and its cluster with a Cl- counterion [M.Cl]+-and a number of their primary fragments were established and systematized. Differences in the pathways of fragmentation of M2+ and [M.Cl]+ were revealed: the main directions of [M.Cl]+ decay involve dequaternization similar to thermal degradation of this compound, while in M2+ fragmentation via loss of one and two terminal radicals with preservation of the doubly charged state of the fragments dominates over charge separation processes. It was shown that pairing of the dication with a Cl- anion does not preserve the complex from fragmentation via separation of two positively charged centers or neutralization (dequaternization) of one such center. At the same time the low abundance of M2+ in the SIMS spectra is to a larger extent controlled by a probability of M2+ association with an anion than by the decay of the dication per se. Quantum chemical calculations of the structural and electronic parameters of the decamethoxinum dication have revealed at least three features which can provide stabilization of the doubly charged state. Firstly, in the most energetically favorable stretch conformation the distance between the quaternary nitrogens (rN1-N2=1.39 nm) is relatively large. Secondly, an intramolecular solvation of quaternary groups by carbonyl oxygens of the adjacent groups of the dication occurs, which contribute to structural stabilization. Thirdly, an important feature of the electronic structure of the dication is the presence of a partial negative charge on the nitrogen atoms and smearing of a positive charge mainly over the hydrogens of alkyl groups attached to the quaternary nitrogens, which reduces the net repulsion between the quaternary groups. The possible influence of charge smearing on the kinetic energy released on the dication fragmentation is discussed.
在对双季铵抗菌剂十甲氯铵进行液体二次离子质谱（SIMS）研究的过程中，观察到了这种双季铵盐的有机二价阳离子M2+的异常碎片化途径，碎片保持了双电荷状态。为了揭示十甲氯铵的结构和电子参数，这些参数负责其有机二价阳离子在气相中的稳定性，我们进行了一项全面的SIMS研究，使用亚稳衰变、碰撞诱导解离和动能释放技术，并辅以从头算量子化学计算。确定并系统化了源自十甲氯铵有机二价阳离子M2+及其与Cl-抗衡离子的簇[M.Cl]+的两种主要前体及其一些初级碎片的碎片化途径。揭示了M2+和[M.Cl]+碎片化途径的差异：[M.Cl]+衰变的主要方向涉及去季铵化，类似于该化合物的热降解，而在M2+碎片化中，通过失去一个和两个末端自由基并保持碎片的双电荷状态，比电荷分离过程占主导地位。结果表明，二价阳离子与Cl-阴离子的配对不能防止配合物通过两个带正电中心的分离或一个这样的中心的中和（去季铵化）而碎片化。同时，SIMS光谱中M2+的低丰度在很大程度上受M2+与阴离子缔合概率的控制，而不是受二价阳离子本身衰变的控制。十甲氯铵二价阳离子的结构和电子参数的量子化学计算揭示了至少三个可以提供双电荷状态稳定性的特征。首先，在能量最有利的伸展构象中，季氮之间的距离（rN1-N2 = 1.39 nm）相对较大。其次，二价阳离子相邻基团的羰基氧对季铵基团进行分子内溶剂化，这有助于结构稳定。第三，二价阳离子电子结构的一个重要特征是氮原子上存在部分负电荷，正电荷主要分布在与季氮相连的烷基的氢上，这减少了季铵基团之间的净排斥力。讨论了电荷分布对二价阳离子碎片化时释放的动能的可能影响。
