Department of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, SE-106 91, Stockholm, Sweden.
School of Chemical Engineering and Technology, Sun Yat-sen University, Zhuhai, 519082, P. R. China.
Chemphyschem. 2020 Jun 3;21(11):1202-1214. doi: 10.1002/cphc.201901206. Epub 2020 Apr 21.
Extensive atomistic simulations demonstrated that a gradual substitution of hexyl chains with phenyl groups in tetraalkylphosphonium cations results in remarkable changes in hydrogen bonding interactions, liquid structures and scattering structural functions, and rotational dynamics of hexyl chains and phenyl groups in tetraalkylphosphonium bis(trifluoromethylsulfonyl)imide ionic liquids. Hydrogen donor sites in hexyl chains present competitive characteristics with those in phenyl groups in coordinating anions, as well as their continuous and intermittent hydrogen bonding dynamics. Cation-cation and anion-anion spatial correlations show concomitant shift to short distances with decreased peak intensities with variations of cation structures, whereas cation-anion correlations have a distinct shift to large radial distances due to decreased associations of anions with neighboring cations. These microstructural changes are qualitatively manifested in shifts of prominent peaks for prevalent charge alternations and adjacency correlations between ion species in scattering structural functions. Meanwhile, rotational dynamics of hexyl chains speed up, which, in turn, slow down rotations of phenyl groups, whereas anions exhibit imperceptible changes in their rotational dynamics. These computational results are intrinsically correlated with conformational flexibilities, molecular sizes, and steric hindrance effects of phenyl groups in comparison with hexyl chains, and constrained distributions of anions around cations in heterogeneous ionic environments.
通过广泛的原子模拟，我们发现四烷基膦阳离子中逐渐用苯基取代己基链会导致氢键相互作用、液体结构和散射结构函数以及四烷基膦双（三氟甲烷磺酰基）酰亚胺离子液体中己基链和苯基的旋转动力学发生显著变化。己基链中的供氢位点与阴离子中的供氢位点具有竞争性，同时还具有连续和间歇的氢键动力学。随着阳离子结构的变化，阳离子-阳离子和阴离子-阴离子的空间相关性表现出向短距离和低强度峰值的共同移动，而阳离子-阴离子相关性由于阴离子与相邻阳离子的缔合减少而明显向大径向距离移动。这些微观结构的变化在散射结构函数中离子物种之间普遍的电荷交替和相邻相关性的主要峰的移动中得到定性体现。同时，己基链的旋转动力学加快，这反过来又减缓了苯基的旋转，而阴离子的旋转动力学几乎没有变化。这些计算结果与构象灵活性、分子大小以及苯基与己基链相比的空间位阻效应有关，并且与异质离子环境中阴离子在阳离子周围的约束分布有关。
