Alkan Melisa, Rogachev Andrey Yu
Department of Chemistry, Illinois Institute of Technology, Chicago, IL 60616, USA.
Phys Chem Chem Phys. 2020 Mar 28;22(12):6716-6726. doi: 10.1039/c9cp06935f. Epub 2020 Mar 12.
In this study, a comprehensive theoretical investigation of both kinetic and thermodynamic stabilities was performed for dimeric dianionic systems (CH) and (CH), neutralized by two alkali metal cations. The influence of the counterions was of primary interest. The impact of the additional/spectator ligand(s) was elucidated by considering adducts with four molecules of diglyme or two molecules of 18-crown-6 ether. Importantly, both types of systems - in the form of contact-ion pair (CIP) and solvent-separated ion pair (SSIP) - were considered. The SSIP set was augmented by the adduct, in which the dimeric dianionic species were neutralized with purely organic cations N(CH) and P(CH). Detailed analysis of the bonding revealed that the presence of the counterions made these systems thermodynamically stable. This finding is in sharp contrast with results obtained for isolated (PAH) systems, which were previously found to be thermodynamically unstable, but kinetically persistent. The introduction of the alkali metal cations to the system significantly increases the ionic term (ΔE), whereas the repulsive ΔE one was found to be substantially reduced. Considering that the orbital component (ΔE) exhibited only a moderate decrease and the preparation energy (ΔE) showed no changes, the above-mentioned changes in ΔE and ΔE provided a clear explanation for the increase of the thermodynamic stability of the target species. Importantly, a clear correlation between the size of the alkali metal cation and stability of the target dimeric product was established. Thermodynamic stability of the system rises with a decrease in the size of M due to enlargement of the ΔE. Evaluated energy barriers (as spin-crossing points between singlet and triplet energy surfaces) were found to be equal to +15.85 kcal mol and +18.5 kcal mol for [(Cs){(CH)}] and [(Cs){(CH)}], respectively, which is substantially higher than those calculated for isolated (PAH) systems (+10.00 kcal mol for (CH) and +12.35 kcal mol for (CH)). Thus, this study identified the presence of counterions as the key factor, which have a dramatic influence on the thermodynamic and kinetic stabilities of the aimed dianionic dimeric systems, which are formed by two curved polyaromatic monoanion-radicals.
在本研究中，对由两个碱金属阳离子中和的二聚双阴离子体系(CH)和(CH)的动力学和热力学稳定性进行了全面的理论研究。主要关注抗衡离子的影响。通过考虑与四个二甘醇二甲醚分子或两个18-冠-6醚分子形成的加合物，阐明了额外/旁观配体的影响。重要的是，考虑了两种类型的体系——接触离子对(CIP)和溶剂分隔离子对(SSIP)。SSIP组通过加合物得到扩充，其中二聚双阴离子物种用纯有机阳离子N(CH)和P(CH)中和。对键合的详细分析表明，抗衡离子的存在使这些体系具有热力学稳定性。这一发现与孤立的(PAH)体系的结果形成鲜明对比，后者先前被发现热力学不稳定，但动力学上持久。将碱金属阳离子引入体系显著增加了离子项(ΔE)，而排斥性的ΔE1被发现大幅降低。考虑到轨道分量(ΔE)仅适度下降且制备能(ΔE)没有变化，上述ΔE和ΔE的变化为目标物种热力学稳定性的增加提供了清晰的解释。重要的是，建立了碱金属阳离子大小与目标二聚产物稳定性之间的明确相关性。由于ΔE增大，体系的热力学稳定性随着M尺寸的减小而升高。发现[(Cs){(CH)}]和[(Cs){(CH)}]的评估能垒(作为单重态和三重态能量表面之间的自旋交叉点)分别等于+15.85 kcal mol和+18.5 kcal mol，这大大高于为孤立的(PAH)体系计算的值((CH)为+10.00 kcal mol，(CH)为+12.35 kcal mol)。因此，本研究确定抗衡离子的存在是关键因素，其对由两个弯曲的多芳族单阴离子自由基形成的目标双阴离子二聚体系的热力学和动力学稳定性有显著影响。
