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.

有时会出现氢 (H) 键是否比等效的氘 (D) 键强或弱的问题。蛋白质折叠动力学、大气化学反应或溶剂化行为受到这些相互作用的不同强度的强烈影响。电子相同，差异仅与质量有关，主要是由于振动频率的变化，通常有利于中性和离子水复合物中的 D 键，但零点振动的效果相反。能量(ZPVE) 稳定了水二聚体中的 D 键，而削弱了阳离子 Zundel-Ion 中的键。这种情况在溶液中变得更加难以分析，其中周围溶剂分子的“浴”可能会改变对 D 键的偏好。对于离子液体 (IL)，氘化的影响几乎是未知的，尽管这些材料可以在带相反电荷和同电荷离子之间显示出各种各样的 H/D 键合，因此可以作为研究的模型系统同位素效应取决于所涉及成分的电荷，在这里，我们选择了羧基功能化的离子液体，它能够形成三种不同类型的 H/D 键：阳离子之间的双 H/D 键和单 H/D 键。阳离子和阴离子之间的 D 键 H 和 D 键的强度很大程度上取决于离子之间的吸引力或排斥力。通过增加烷基链长度，我们可以减少排斥力，同时增强 H 力。这些离子配合物中的同位素效应应该与苯基链烷酸中的同位素效应相似，苯基链烷酸是此处使用的离子液体阳离子的分子模拟物，我们通过红外和核磁共振光谱、中子衍射和低温离子来解决以下问题。气相振动 (CIV) 光谱，以及支持 DFT 计算：IL 和苯基的羧基中的 D 键是否存在链烷酸比相应的 H 键强，为什么 H 或 D 键的电荷是正还是负重要？熵和温度如何影响 H 和 D 键的相对稳定性？最后，如果阳离子在不参与氢键的不同位置进行氘化，对结构和动力学的影响有多大？我们的研究将为同位素效应提供新的线索，并解释它们如何从离子转变为离子。分子液体。