Abstract:

Three protic ionic liquids (PILs) composed of the same anion of [TFSA== (CF₃SO₂)₂N^-] and different cations of N-hexylammonium (HHexam⁺), monoprotic hexylethylenediaminium (HHexen⁺) and hexyldiethylenetriaminium (HHexdien⁺), [HHexam][TFSA], [HHexen][TFSA], and [HHexdien][TFSA] were studied for the absorption of CO₂. First, the more stable configurations of the three PILs were optimized via the M06-2X/6-311G(d,p) of the density functional theory. The results indicated that the stronger N—H···N-type hydrogen bonds were formed mainly between the N-atoms in the cations and the N-atoms in the anion of the PILs. Then, the configurations of PIL-nCO₂ were optimized. The N—H···O-type weak- or moderate-strength hydrogen bonds were formed mainly between the N—H bond in the cation of the PIL and the O atoms of CO₂. The results of the vibrational frequency of the N—H bond, and the electron density and the second-order perturbation energy of N—H···O showed that a single molecule of [HHexam][TFSA], [HHexen][TFSA] and [HHexdien][TFSA] was saturated when bonded with 2,3 and 4 CO₂ molecules, respectively. Meanwhile, the results calculated by COSMOtherm software found that the Henry constants (kPa) for CO₂ in the three PILs varied as 1.91×10⁴ for [HHexam][TFSA]>1.68×10⁴ for [HHexen][TFSA]>1.51×10⁴ for [HHexdien][TFSA], indicating that the solubility of CO₂ in the PILs followed the order of [HHexdien][TFSA] with three amino groups in the polar head > [HHexen][TFSA] with two amino groups > [HHexam][TFSA] with one amino group. These results suggested that the number of amino groups in the PIL structure had a significant effect on its ability to absorb CO₂. With increasing number of amino groups in the structure of PILs, its solubilization capacity for CO₂ increased.

Key words: protic ionic liquids with polyamines, carbon dioxide, intermolecular hydrogen bonding, density functional theory (DFT), atoms in molecules (AIM)

摘要：

开展了以质子化的正己胺（HHexam^＋)、己基乙二胺（HHexen^＋）及己基二亚乙基三胺（HHexdien^＋）为阳离子的TFSA [== (CF₃SO₂)₂N^-]型质子化离子液体（PILs），即[HHexam][TFSA]、[HHexen][TFSA]及[HHexdien][TFSA]型PILs吸收CO₂的研究。首先，选择密度泛函理论，在M06-2X/6-311G(d, p)水平下，对上述3种PILs的构型进行优化，分别得到了其较稳定构象，结果显示，PILs的阳离子中N—H和阴离子中N原子间主要形成N—H···N型较强氢键。然后，分别利用其中最稳定构象，创建并优化PILs-nCO₂构型，PILs和nCO₂分子间主要形成N—H···O型弱或中等强度氢键。主要氢键部位N—H···O中N—H键的振动频率的变化值、电子密度值及二阶微扰能的计算结果显示，[HHexam][TFSA]、[HHexen][TFSA]及[HHexdien][TFSA]分别与2、3、4分子CO₂结合时将不再形成氢键网络。采用COSMOtherm软件计算的CO₂在3种PILs中的亨利常数（kPa）大小为[HHexam][TFSA] (1.91×10⁴) > [HHexen][TFSA] (1.68×10⁴) > [HHexdien][TFSA] (1.51×10⁴)，即3种PILs对CO₂的溶解能力大小为极性头部具有3个氨基的[HHexdien][TFSA] > 2个氨基的[HHexen][TFSA] > 1个氨基的[HHexam][TFSA]。以上结果中可以看出，PILs结构中氨基数目的多少对其吸收CO₂的能力有较显著影响，即随着PILs结构中氨基数目的增多，其对CO₂的溶解能力随之增大。

关键词: 多元胺型质子化离子液体, 二氧化碳, 分子间氢键作用, 密度泛函理论, 分子中原子理论