Department of Physical and Theoretical Chemistry, School of Chemistry, University of Nottingham, University Park, Nottingham NG7 2RD, United Kingom.
J Phys Chem A. 2013 May 16;117(19):3877-86. doi: 10.1021/jp311950p. Epub 2013 May 6.
A series of five molecular dication clusters, (H2O)n(2+), (NH3)n(2+), (CH3CN)n(2+), (C5H5N)n(2+), and (C6H6)n(2+), have been studied for the purpose of identifying patterns of behavior close to the Rayleigh instability limit where the clusters might be expected to exhibit Coulomb fission. Experiments show that the instability limit for each dication covers a range of sizes and that on a time scale of 10(-4) s ions close to the limit can undergo either Coulomb fission or neutral evaporation. The observed fission pathways exhibit considerable asymmetry in the sizes of the charged fragments, and are associated with kinetic (ejection) energies of ~0.9 eV. Coulomb fission has been modeled using a theory recently formulated to describe how charged particles of dielectric materials interact with one another (Bichoutskaia et al. J. Chem. Phys. 2010, 133, 024105). The calculated electrostatic interaction energy between separating fragments accounts for the observed asymmetric fragmentation and for the magnitudes of the measured ejection energies. The close match between theory and experiment suggests that a significant fraction of excess charge resides on the surfaces of the fragment ions. The experiments provided support for a fundamental step in the electrospray ionization (ESI) mechanism, namely the ejection from droplets of small solvated charge carriers. At the same time, the theory shows how water and acetonitrile may behave slightly differently as ESI solvents. However, the theory also reveals deficiencies in the point-charge image-charge model that has previously been used to quantify Coulomb fission in the electrospray process.
已经研究了一系列五个分子二价阳离子簇，（H2O）n（2+），（NH3）n（2+），（CH3CN）n（2+），（C5H5N）n（2+）和（C6H6）n（2+），目的是确定接近瑞利不稳定性极限的行为模式，在这种情况下，簇可能表现出库仑裂变。实验表明，每个二价阳离子的不稳定性极限覆盖了一系列尺寸，并且在 10（-4）s 的时间尺度上，接近极限的离子可以经历库仑裂变或中性蒸发。观察到的裂变途径在带电碎片的尺寸上表现出相当大的不对称性，并且与~0.9eV 的动力学（喷射）能量相关。库仑裂变已使用最近提出的描述介电材料中带电粒子如何相互作用的理论进行建模（Bichoutskaia 等人，J. Chem. Phys. 2010，133，024105）。分离碎片之间的计算静电相互作用能解释了观察到的不对称分裂以及测量的喷射能量的大小。理论与实验的紧密匹配表明，多余的电荷有很大一部分位于碎片离子的表面上。该实验为电喷雾电离（ESI）机制中的一个基本步骤提供了支持，即从小的溶剂化电荷载体液滴中喷射。同时，该理论表明水和乙腈在作为 ESI 溶剂时可能会略有不同。然而，该理论也揭示了先前用于量化电喷雾过程中库仑裂变的点电荷图像电荷模型的缺陷。
