Ab Initio Simulations of Poorly- and Well-Equilibrated (CH3CN)n¯ Cluster Anions: Assigning Experimental Photoelectron Peaks to Surface-Bound Electrons and Solvated Monomer and Dimer Anions
Ab Initio Simulations of Poorly- and Well-Equilibrated (CH3CN)n¯ Cluster Anions: Assigning Experimental Photoelectron Peaks to Surface-Bound Electrons and Solvated Monomer and Dimer Anions
复制标题
平衡不良和平衡良好的 (CH3CN)n 簇阴离子的从头算：将实验光电子峰分配给表面结合电子以及溶剂化单体和二聚体阴离子
DOI：
10.1021/acs.jpca.1c05855
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发表时间：
2021
期刊：
影响因子：
--
通讯作者：
Schwartz, B. J.
中科院分区：
文献类型：
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作者：
Narvaez, W. A.;Schwartz, B. J.
液体乙腈中的过量电子特别令人感兴趣，因为它们在平衡中以两种不同的形式存在：它们可以作为传统的溶剂化电子存在于空腔中，并且它们可以形成某种类型的溶剂化分子阴离子。小乙腈簇阴离子在气相中的研究表明，两种异构体具有不同的垂直脱离能，它是诱人的假设，这两个气相簇阴离子异构体的两个过剩的电子物种存在于本体溶液中的前体。在本文中，我们进行DFT basedab initiomolecular动力学模拟乙腈簇阴离子，以了解电子物种存在，为什么他们有不同的结合能。使用一个远程校正的密度泛函，最佳调谐到描述乙腈簇阴离子结构，我们从理论上探讨了化学（CH 3CN）n-簇阴离子与sizesn= 5，7，和10。由于实验团簇阴离子的温度未知，我们进行了两组模拟，研究了制备团簇阴离子的方式如何影响过量电子结合基序：一组模拟简单地将过量电子附着到中性（CH 3CN）n团簇上，在过量电子的存在下为团簇提供很少的松弛机会，而另一组允许簇阴离子在室温附近热平衡。我们发现，这两组模拟显示三个不同的电子结合图案：电子可以连接到簇的表面（偶极结合）或存在的溶剂化单体阴离子，CH 3CN-，或溶剂化分子二聚体阴离子，（CH 3CN）2-。所有这三种物质在较大的簇尺寸下具有较高的结合能。热平衡强烈有利于形成的价键结合的分子阴离子相对于表面结合的过量电子，和二聚体阴离子变得比单体阴离子和表面结合的物种更稳定的簇尺寸的增加。从我们的模拟中，有差的热平衡计算的光电子能谱与实验吻合得很好，这表明分配的两个实验簇阴离子异构体的表面结合电子和溶剂化的分子二聚体阴离子。模拟还表明，实验中在低能异构体的脱离峰上看到的肩并不是振动过程的一部分，而是由分子单体阴离子引起的。在我们探索的尺寸范围内，我们没有看到非价、腔束缚的电子溶剂化的证据，这表明这种物质可能只在具有良好热平衡的较大尺寸下才能接触到。
Excess electrons in liquid acetonitrile are of particular interest because they exist in two different forms in equilibrium: they can be present as traditional solvated electrons in a cavity, and they can form some type of solvated molecular anion. Studies of small acetonitrile cluster anions in the gas phase show two isomers with distinct vertical detachment energies, and it is tempting to presume that the two gas-phase cluster anion isomers are precursors of the two excess electron species present in bulk solution. In this paper, we perform DFT-basedab initiomolecular dynamics simulations of acetonitrile cluster anions to understand the electronic species that are present and why they have different binding energies. Using a long-range-corrected density functional that was optimally tuned to describe acetonitrile cluster anion structures, we have theoretically explored the chemistry of (CH3CN)n–cluster anions with sizesn= 5, 7, and 10. Because the temperature of the experimental cluster anions is not known, we performed two sets of simulations that investigated how the way in which the cluster anions are prepared affects the excess electron binding motif: one set of simulations simply attached excess electrons to neutral (CH3CN)nclusters, providing little opportunity for the clusters to relax in the presence of the excess electron, while the other set allowed the cluster anions to thermally equilibrate near room temperature. We find that both sets of simulations show three distinct electron binding motifs: electrons can attach to the surface of the cluster (dipole-bound) or be present either as solvated monomer anions, CH3CN–, or as solvated molecular dimer anions, (CH3CN)2–. All three species have higher binding energies at larger cluster sizes. Thermal equilibration strongly favors the formation of the valence-bound molecular anions relative to surface-bound excess electrons, and the dimer anion becomes more stable than the monomer anion and surface-bound species as the cluster size increases. The calculated photoelectron spectra from our simulations in which there was poor thermal equilibration are in good agreement with experiment, suggesting assignment of the two experimental cluster anion isomers as the surface-bound electron and the solvated molecular dimer anion. The simulations also suggest that the shoulder seen experimentally on the low-energy isomer’s detachment peak is not part of a vibronic progression but instead results from molecular monomer anions. Nowhere in the size range that we explore do we see evidence for a nonvalence, cavity-bound interior-solvated electron, indicating that this species is likely only accessible at larger sizes with good thermal equilibration.