The role of isotope effects: From ionic to molecular liquids

有时会出现这样的问题，即氢键（H）是否比等价的氘键（D）更强或更弱。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。氢和氘在电子上是相同的，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D键。然而，对于中性和离子水络合物中的H和D键，效果相反。零点振动能（ZPVE）稳定了水二聚体中的D键，而它们削弱了阳离子Zundel离子中的键。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。离子液体（IL）的氘代效应几乎是未知的，尽管这些材料可以在相反和相同电荷的离子之间显示出各种各样的H/D键，因此可以作为研究同位素效应的模型系统，这取决于所涉及的组分的电荷。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：阳离子之间的双和单H/D键，以及阳离子和阴离子之间的单H/D键。H和D键的强度强烈地依赖于离子之间的吸引或排斥库仑力。通过增加烷基链的长度，我们可以降低库仑排斥力，同时加强H和D键。这些离子络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，苯基链烷酸是这里使用的IL阳离子的分子模拟物。我们解决了以下问题，通过红外光谱和核磁共振光谱，中子衍射和低温离子振动（CIV）光谱在气相中，沿着与支持DFT计算：是D键的离子液体和苯基链烷酸的羧基比相应的H键强，为什么？H键或D键的电荷是正的还是负的重要吗？协同效应如何影响H键和D键的相对稳定性？熵和温度如何影响H和D键？最后，如果阳离子在不参与氢键的不同位置被氘化，结构和动力学受到多大的影响？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液体转变为分子液体。

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.