The role of isotope effects: From ionic to molecular liquids

同位素效应的作用：从离子液体到分子液体

• 批准号: 517661181
• 负责人: Professor Dr. Ralf Ludwig
• 金额: --
• 依托单位: Lehrstuhl für Allgemeine Physikalische und Theoretische Chemie
• 依托单位国家: 德国
• 项目类别: Research Grants
• 资助国家: 德国
• 项目状态: 未结题
• 来源: https://gepris.dfg.de/gepris/projekt/517661181?language=en
• 关键词: role; isotope; effects; ionic; molecular

The question sometimes arises whether a hydrogen (H) bond is stronger or weaker than the equivalent deuterium (D) bond. Protein folding dynamics, reactions in atmospheric chemistry or solvation behaviour are strongly affected by the different strength of these interactions. Hydrogen and deuterium are electronically identical and the differences are solely associated with the masses, primarily due to the changing vibrational frequencies, often favoring the D bond. For H and D bonds in neutral and ionic water complexes the effect is, however, the opposite. Zero-point vibrational energies (ZPVE) stabilize the D bonds in the water dimer, whereas they weaken the bonds in the cationic Zundel-Ion. The situation becomes more difficult to analyse in solution, where the “bath” of surrounding solvent molecules might alter the preference for D bonding.The effect of deuteration is almost unknown for ionic liquids (ILs), although these materials can show a broad variety of H/D bonding between opposite- and like-charged ions, and thus might serve as a model system for studying isotope effects depending on the charge of the involved constituents. Here, we chose carboxyl-functionalized ionic liquids, which are capable of forming three different types of H/D bonds: double and single H/D bonds between the cations, and single H/D bonds between cation and anion. The strength of H and D bonds strongly depends on the attractive or repulsive Coulomb forces between the ions. By increasing the alkyl chain length, we can reduce the repulsive Coulomb forces, while strengthening the H and D bonds. The isotope effects in these ionic complexes should then be similar to those in phenyl alkanoic acids, the molecular mimics of the IL cations used here. We address the following questions by means of IR and NMR spectroscopy, neutron diffraction and cryogenic ion vibrational (CIV) spectroscopy in the gas phase, along with supporting DFT calculations: Is the D bond in the carboxyl groups of the ILs and the phenyl alkanoic acids stronger than the corresponding H bonds and why? Does it matter whether the charge of the H or D bonded species is positive or negative? How is the relative stability of the H and D bonds affected by cooperativity? How does entropy and temperature influence H and D bonding? Finally, how strongly are structure and dynamics affected, if the cations are deuterated at different positions that are not involved in hydrogen bonding? Our studies will shed new light on the isotope effects and explain how they change from an ionic to a molecular liquid.

这个问题有时会出现氢（H）键是否比等效氘（D）键更强或弱。蛋白质折叠动力学，大气化学或溶液行为的反应受这些相互作用的不同强度的强烈影响。氢和氘在电子上是相同的，差异完全与质量相关，这主要是由于振动频率变化，通常有利于D键。对于中性和离子水复合物中的H和D键，效果恰恰相反。零点振动能（ZPVE）稳定了水二聚体中的D键，而它们却弱阳离子Zundel-Ion中的键。在解决方案中，情况变得更加难以分析，在这种溶液中，周围溶液分子的“沐浴”可能会改变对D键的偏爱。剥离的效果几乎是离子液体（ILS）的未知的，尽管这些材料可以显示出相反离子和类似的离子之间的众多H/D键合，因此可能会作为模型系统对构图的构成效应，依赖于依赖于效果的模型系统。在这里，我们选择羧基功能化的离子液体，它们能够形成三种不同类型的H/D键：阳离子之间的双重和单h/d键，以及阳离子和阴离子之间的单个H/D键。 H和D键的强度在很大程度上取决于离子之间的吸引力或排斥性库仑力。通过增加烷基链长度，我们可以减少排斥力的库仑力，同时增强H和D键。然后，这些离子复合物中的同位素效应应与苯基烷酸中的苯基烷酸相似，苯基烷烷酸是此处使用的IL阳离子的分子模拟物。我们通过IR和NMR光谱，中子衍射和气相中的低温离子振动（CIV）光谱法解决以下问题，以及支持DFT计算：ILS羧基的D键和苯基烷基烯基的苯基烷基酸是否比相应的H键强？ H或D键合物种的电荷是正还是负面的？ H和D键的相对稳定性如何受协调影响？熵和温度如何影响H和D键？最后，如果阳离子在不涉及氢键的不同位置剥离，则结构和动力学有多强大？我们的研究将为同位素效应提供新的启示，并解释它们如何从离子变为分子液体。