Matthews Richard P, Welton Tom, Hunt Patricia A
Department of Chemistry, Imperial College London, London, SW7 2AZ, UK.
Phys Chem Chem Phys. 2015 Jun 14;17(22):14437-53. doi: 10.1039/c5cp00459d.
A systematic electronic structure analysis of hydrogen bonding (H-bonding), anion-π(+) and π(+)-π(+) interactions present in [C1C1im]Cl ion-pairs (IPs) and selected [C1C1im]2Cl2 IP-dimers has been carried out. Interactions have been characterised using a combination of QTAIM, NCIPLOT, NBO and qualitative MO theory. IP-dimers form non-directional charge quadrupolar arrangements due to Coulombic interactions. These are found to associate either as clusters or as loosely associated IP-IP structures. Large conformational changes are found to occur for very little cost in energy, indicating that charge screening is essentially independent of the cation ring orientation. H-bond formation is accompanied by charge transfer and polarisation of the entire C1C1im ring. Charge transfer does not follow the same trend for the CHelpG, QTAIM and NBO methods. Weak "stacked" π(+)-π(+) interactions are stabilised in the presence of anions, which locate between and at the periphery of the rings, novel strongly bent H-bonds are also present. Primary (ring; C-H···Cl(-)) H-bonds and anion-π(+) (C(2)···Cl(-)) interactions are found to decay more rapidly with distance than secondary (aliphatic; C(M)-H···Cl(-)) H-bonds. This leads to an increase in the relative importance of secondary H-bond interactions in the IP-dimers. Moreover, rotation of the methyl groups within the "stacked" π(+)-π(+) IP-dimers facilitates the formation of (stronger) linear secondary H-bonds. Thus, compared to isolated IPs, secondary H-bonds may play an increased role within the condensed phase. Overall we find that structural fluidity is facilitated by fluctuating hydrogen bond, π(+)-π(+) and anion-π(+) interactions.
已对[C1C1im]Cl离子对（IPs）和选定的[C1C1im]2Cl2 IP二聚体中存在的氢键（H键）、阴离子-π(+)和π(+)-π(+)相互作用进行了系统的电子结构分析。使用QTAIM、NCIPLOT、NBO和定性分子轨道理论相结合的方法对相互作用进行了表征。由于库仑相互作用，IP二聚体形成非定向电荷四极排列。发现这些排列以簇或松散关联的IP-IP结构形式存在。发现发生非常小的能量成本就会出现大的构象变化，这表明电荷屏蔽基本上与阳离子环取向无关。H键的形成伴随着整个C1C1im环的电荷转移和极化。对于CHelpG、QTAIM和NBO方法，电荷转移并不遵循相同的趋势。在阴离子存在的情况下，弱的“堆叠”π(+)-π(+)相互作用得到稳定，阴离子位于环之间和环的外围，还存在新型的强弯曲H键。发现初级（环；C-H···Cl(-)）H键和阴离子-π(+)（C(2)···Cl(-)）相互作用随距离的衰减比次级（脂肪族；C(M)-H···Cl(-)）H键更快。这导致次级H键相互作用在IP二聚体中的相对重要性增加。此外，“堆叠”π(+)-π(+) IP二聚体内甲基的旋转促进了（更强的）线性次级H键的形成。因此，与孤立的IPs相比，次级H键在凝聚相中可能发挥更大的作用。总体而言，我们发现结构流动性是由波动的氢键、π(+)-π(+)和阴离子-π(+)相互作用促进的。
