甘氨酸咪唑型离子液体的耦合机制及相关性质的研究

【作者】 牟朝霞；

【导师】 步宇翔；

【作者基本信息】 曲阜师范大学 ， 物理化学， 2008， 硕士

【摘要】 近年来，室温离子液体（RTIL）的研究已经激起人们很大的兴趣，离子液体的设计和应用已经发展成为化学中的一个很重要的分支。离子液体具有许多独特的性质，包括低蒸汽压、高热稳定性和高离子传导性等，在作为反应溶剂、萃取溶剂和电解热材料方面引起人们极大的关注，被认为是一种新型的、绿色的溶剂。而且，为了某些特定的用途，可以通过调制阳离子或阴离子的结构来设计其物理性质，因此，离子液体又被称为“可设计的溶剂”。现已广泛应用于有机合成、催化反应、电化学、生物化学和材料设计等许多领域。本文我们讨论了一种新型的离子液体，即由1-乙基-3-甲基咪唑阳离子和甘氨酸阴离子构成的离子液体（[emim][Gly]），该离子液体区别于传统离子液体的是，阴离子为与生命过程密切相关的甘氨酸阴离子。在离子液体的理论研究方面，对该体系的研究尚属首次。作为一类完全由离子构成的介质，离子液体具有与传统分子介质不同的内部环境。微观结构决定性质，对离子液体这种离子介质中各物质的相互作用和性质的研究有助于获得一些有别于分子介质的规律与知识。本论文从研究离子液体的内部相互作用出发，利用密度泛函理论B3LYP／6-31+G^*和B3LYP／6-311++G^**方法对甘氨酸离子液体的构型和性质进行了一系列的讨论，并取得了一些非常有意义的结论：首先，我们对由甘氨酸阴离子和1-乙基-3-甲基咪唑阳离子组成的离子液体体系的结构进行了研究。结果表明，甘氨酸阴离子的羰基氧与咪唑环的C2-H和咪唑阳离子的甲基侧链的C-H相互作用时，相互作用模式是最为有利的。所有的稳定构型均以分子间氢键为特征，通过双氢键或单氢键相互作用，通过红外光谱分析也可以进一步证实。与相互作用能的顺序一致，得到了所有构型中最稳定的构型。通过对离子液体稳定构型的红外光谱分析，不同区域出现的吸收强度可以为不同复合物的鉴定提供帮助。而且，离子对的形成主要影响了咪唑阳离子的C-H部分和甘氨酸阴离子的C=O部分的振动，其它振动模式变化不大，本质上没有相互混淆。其次，通过NBO（Natural Bond orbital）和AIM（Atom in Molecule）分析，我们对阴阳离子间氢键作用的化学本质进行了研究。结果表明，在氢键形成的过程中，[Gly]^-的氧原子的孤对电子向[emim]⁺的C-H反键轨道上的转移导致了[emim]⁺的C-H键的伸长和振动频率的红移。而且，C-H键的伸长与相应的反键轨道σ^*_C-H的电子密度分布几乎为线性相关。阴离子的孤对电子和C-H反键轨道之间的n→σ^*_C-H作用能够提供较大的稳定化能，它对整个体系的稳定性起到了非常关键的作用。实验上观察到的氨基酸离子液体的高稳定性应该与不存在质子转移产物（中性分子）和气相中离子液体的解离需要很大能量是相关的。最后，我们讨论了H⁺、Li⁺、Na⁺和K⁺的引入对所研究离子液体体系的影响。上述阳离子与甘氨酸阴离子的O22位能形成更稳定的复合物。引入阳离子后，离子液体内起主要作用的分子间氢键的强度均减弱，相应的C-H键发生蓝移，强度增强。四种阳离子与离子液体相互作用的顺序为H⁺>Li⁺>Na⁺>K⁺。其中H⁺与[emim][Gly]的耦合更多的倾向为共价键特征，其相互作用最强，而Li⁺、Na⁺、K⁺与[emim][Gly]的耦合倾向为静电作用。引入四种阳离子后，离子液体仍不能发生质子转移，仍保持原来的作用模式。H⁺、Li⁺、Na⁺和K⁺阳离子与[emim][Gly]复合物的红外光谱以C=O和咪唑环上的C-H键的振动为特征，而且阳离子的引入在一定程度上也影响了离子对的其它键的振动。更多还原

【Abstract】 In recent years, there have been a great enthusiasm for room temperature ionic liquids （RTIL） and their design and use have evolved into a blossoming branch of chemistry. Ionic liquids （ILs） are perceived to be a novel and "green" solvents because they are nonvolatile and their quite remarkable properties, including a negligibly small vapor pressure, high thermal stability, and high ionic conductivity. As a result, ILs are attracting considerable attention as reaction solvents, extraction solvents, and electrolyte materials. Moreover, ILs are now expected to be "designed solvents" because their physical properties could be tailored by adjusting the structures and species of cations and/or anions for a given end use. Therefore, much of ILs has been used in wide fields, for example, organic synthesis and catalytic reaction, electrochemistry, biochemistry, and material engineering.In this study, novel ionic liquids formed by 1-ethyl-3-methyl-imidazolium cation [emim]⁺ and glycine anion [Gly]^- have been investigated theoretically. The difference of these ionic liquids from the others is that the anion is [Gly]^- which is the basic component of protein and indispensable in the natural living body. This is the first time for the investigation of this question theoretically. As a kind of new ionic solvents, the ionic liquids （ILs） have different interal environments compared with the traditional molecular solvents. A systematic study of the interaction in the ILs can get a better understanding of their structure-activity relationship. In the present work, theoretical investigations of the coupling interactions in the ILs have been carried out employing B3LYP/6-31+G^* and B3LYP/6-311++G^** level of theory. The primary innovations are related as follows.Firstly, the structural characteristics of the ionic liquids formed by 1-ethyl-3-methyl-imidazolium cation [emim]⁺ and glycine anion [Gly]^- have been investigated. The interaction modes are most favorable when the carbonyl O atom of [Gly]^- interacts with the C2-H of the imidazolium ring and the C-H of the methyl group of [emim]⁺ through the formation of double intermolecular H-bonds. All of the stable geometries are characterized by the intermolecular H-bonds, which is further supported by the IR absorption peaks. Consistent with the relative order of interaction energies, the most stable complexes have been determined. In the IR spectra, the different intensities occurring in different regions can provide some help in the identification of various complexes. The formation of the ion pairs mainly influences the vibrations of the imidazolium C-H groups and the C=O of [Gly]^-, and other modes of the ions retain their individuality and practically do not mix.Secondly, the nature of the intermolecular H-bond in the ionic liquids has been investigated employing the natural bond orbital （NBO） and atom in molecular （AIM） methods. The NBO analysis demonstrated that the stabilization energy is due to the n→σ_C-H^* orbital interaction. In the formed intermolecular H-bonds, electron transfers occur mainly from the lone pairs of 0 atom of [Gly]^- to the C-H antibonding orbital of [emim]⁺, resulting in the elongation and red-shift of the C-H stretching frequency. Moreover, an almost linear correlation between the elongation of the C-H bond and the change of the electron population in the correspondingσ^*（C-H） orbital has also been observed. The origin of the high stability of the amino acid ionic liquids observed experimentally may be relevant to the nonexistence of the proton-transferred products （neutral pairs） together with the larger energy needed for separation of the ionic pairs in the gas phase.Finally, the effects of the H⁺, Li⁺, Na⁺ and K⁺ cations on the ionic liquids have been investigated. The complexes are more stable when the H⁺, Li⁺, Na⁺ and K⁺ cations interact with the 022 of the [Gly]^-. The intermolecular H-bonds in the ionic liquids have been weakened, resulting in the blue-shifts of the C-H stretching frequency. The order of the interaction is H⁺>Li⁺>Na⁺>K⁺. The interaction between H⁺ and [emim][Gly] is mainly covalent, and the other interactions between Li⁺, Na⁺ and K⁺ and [emim][Gly] are mainly electrostatic. The proton transfer could not occur among those complexes. The formation of the ion pairs mainly influences the vibrations of the imidazolium C-H groups and the C=O of [Gly]^-, and other modes of the ions have also been influenced in some extent.更多还原