Patil Amol Baliram, Bhanage Bhalchandra Mahadeo
Department of Chemistry, Institute of Chemical Technology, Mumbai, 400019, India.
Phys Chem Chem Phys. 2016 Jun 21;18(23):15783-90. doi: 10.1039/c6cp02819e. Epub 2016 May 27.
The nature of bonding interactions between the cation and the anion of an ionic liquid is at the heart of understanding ionic liquid properties. A particularly interesting case is a special class of ionic liquids known as protic ionic liquids. The extent of proton transfer in protic ionic liquids has been observed to vary according to the interacting species. Back proton transfer renders protic ionic liquids volatile and to be considered as inferior ionic liquids. We try to address this issue by employing modern ab initio valence bond theory calculations. The results indicate that the bonding in the cation and the anion of a prototypical ionic liquid, ethylammonium nitrate, is fundamentally different. It is neither characteristic of covalent/polar covalent bonding nor ionic bonding but rather charge shift bonding as a resonance hybrid of two competing ionic molecular electronic structure configurations. An investigation of other analogous protic ionic liquids reveals that this charge shift bonding seems to be a typical characteristic of protic ionic liquids while the ionic solid analogue compound ammonium nitrate has less charge shift bonding character as compared to protic ionic liquids. Further the extent of charge shift bonding character has been found to be congruent with the trends in many physicochemical properties such as melting point, conductivity, viscosity, and ionicity of the studied ionic liquids indicating that percentage charge shift character may serve as a key descriptor for large scale computational screening of ionic liquids with desired properties.
离子液体中阳离子与阴离子之间键合相互作用的本质是理解离子液体性质的核心。一个特别有趣的例子是一类特殊的离子液体，称为质子离子液体。据观察，质子离子液体中质子转移的程度会根据相互作用的物种而变化。反向质子转移使质子离子液体具有挥发性，因此被认为是较差的离子液体。我们试图通过采用现代从头算价键理论计算来解决这个问题。结果表明，典型离子液体硝酸乙铵的阳离子和阴离子中的键合本质上是不同的。它既不是共价/极性共价键合的特征，也不是离子键合的特征，而是作为两种相互竞争的离子分子电子结构构型的共振杂化体的电荷转移键合。对其他类似质子离子液体的研究表明，这种电荷转移键合似乎是质子离子液体的典型特征，而离子固体类似化合物硝酸铵与质子离子液体相比，电荷转移键合特征较少。此外，已发现电荷转移键合特征的程度与所研究离子液体的许多物理化学性质（如熔点、电导率、粘度和离子性）的趋势一致，这表明电荷转移特征百分比可作为大规模计算筛选具有所需性质的离子液体的关键描述符。
