Department of Chemistry, Faculty of Science, Kyushu University, Hakozaki, Higashi-ku, Fukuoka 812-8581, Japan.
J Phys Chem B. 2010 Sep 16;114(36):11715-24. doi: 10.1021/jp1044755.
The conformational isomerism of the 1-n-butyl-3-methylimidazolium cation, C(4)mim, in halide-based ionic liquids--[C(4)mim]Cl, [C(4)mim]Br, and [C(4)mim]I--was explored by Raman spectroscopy. The C(4)mim cation exhibits trans-gauche conformational isomerism with respect to the N1-C7-C8-C9 dihedral angle of its butyl chain. The thermodynamics of trans-gauche conversion were analyzed through the successful evaluation of the corresponding Gibbs free energy, Δ(iso)G°, enthalpy, Δ(iso)H°, and entropy, Δ(iso)S°, of conformational isomerization. The values of Δ(iso)G° obtained are small (a few units of kJ/mol) and show a slight negative variation with the decrease of the size of the halide anion. On the other hand, Δ(iso)H° and Δ(iso)S° values are positive for [C(4)mim]I and decrease with the anion size to yield negative values for [C(4)mim]Cl and [C(4)mim]Br. This suggests that the negative electrostatic field around the halide anions stabilizes the gauche isomer from an enthalpic point of view. In order to study the structure and ion-ion interactions in this type of ionic liquids, high-energy X-ray diffraction experiments were performed for [C(4)mim]Cl at different temperatures and for supercooled [C(4)mim][Br] at ambient temperature. Molecular dynamics (MD) simulations for these systems were also carried out at several temperatures. Δ(iso)G° and Δ(iso)H° values derived from the simulations qualitatively agree with the experimental ones. Experimental X-ray structure factors are also well reproduced by the simulations. The MD results also allowed the calculation of different spatial distribution functions (SDFs) for the three ionic liquids. Although all SDFs exhibit similar trends, [C(4)mim]I shows a reduced anion density facing the C(2)-H atoms of the cation and enhanced anion densities above and below the imidazolium ring plane. This indicates that anions localized near the C(2)-H atoms of the cation can stabilize their gauche conformer, an effect that is stronger with smaller anions. This conclusion is also supported by ab initio calculations at the CCSD(T) level for isolated ion pairs.
1-正丁基-3-甲基咪唑阳离子C(4)mim在卤化物基离子液体中的构象异构——[C(4)mim]Cl、[C(4)mim]Br 和 [C(4)mim]I——通过拉曼光谱进行了研究。C(4)mim阳离子的正丁基链的 N1-C7-C8-C9 二面角表现出反式- gauche 构象异构。通过成功评估相应的吉布斯自由能、Δ(iso)G°、焓、Δ(iso)H°和熵、Δ(iso)S°，分析了反式- gauche 转化的热力学。得到的Δ(iso)G°值很小（几个 kJ/mol），并且随着卤化物阴离子尺寸的减小而略有负变。另一方面，[C(4)mim]I 的Δ(iso)H°和Δ(iso)S°值为正，并且随着阴离子尺寸的减小而减小，对于 [C(4)mim]Cl 和 [C(4)mim]Br 则产生负值。这表明卤化物阴离子周围的负静电场从焓的角度稳定了 gauche 异构体。为了研究这种类型的离子液体中的结构和离子-离子相互作用，在不同温度下对 [C(4)mim]Cl 进行了高能 X 射线衍射实验，并在环境温度下对过冷的 [C(4)mim][Br]进行了实验。还在几个温度下对这些系统进行了分子动力学 (MD) 模拟。模拟得出的Δ(iso)G°和Δ(iso)H°值与实验值定性一致。模拟还很好地再现了实验 X 射线结构因子。MD 结果还允许为三种离子液体计算不同的空间分布函数 (SDF)。尽管所有 SDF 都表现出相似的趋势，但 [C(4)mim]I 显示出面对阳离子的 C(2)-H 原子的阴离子密度降低，以及在咪唑环平面上方和下方的阴离子密度增强。这表明阴离子局部化在阳离子的 C(2)-H 原子附近可以稳定它们的 gauche 构象，对于较小的阴离子，这种效应更强。这一结论也得到了孤立离子对在 CCSD(T)水平上的从头算计算的支持。
