Possible Proton Conduction Mechanism in Pseudo-Protic Ionic Liquids: A Concept of Specific Proton Conduction
Possible Proton Conduction Mechanism in Pseudo-Protic Ionic Liquids: A Concept of Specific Proton Conduction
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DOI：
10.1021/acs.jpcb.9b03185
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发表时间：
2019-07-25
影响因子：
3.3
通讯作者：
Umebayashi, Yasuhiro
中科院分区：
文献类型：
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作者：
Watanabe, Hikari;Umecky, Tatsuya;Umebayashi, Yasuhiro
在以前的工作中，我们已经发现，赝质子离子液体N-甲基咪唑乙酸盐，[C（1）HIm] [OAc]或[Hmim] [OAc]，主要由电中性分子物种N-甲基咪唑，C（1）Im，和乙酸，AcOH组成，即使该混合物具有显著的离子导电性。采用同位素取代拉曼光谱（ISRS）和脉冲场梯度（PFG）NMR自扩散测量，该系统被重新访问。ISRS和PFG-NMR结果充分证实了我们先前的发现。特别是，在AcOH中的羟基氢原子的自扩散系数是相同的甲基氢原子的实验不确定性内，符合非常少的电离。因此，质子传导机制类似于Grotthuss机制的酸性水溶液被假定为负责所观察到的电导率。根据输运性质计算了Laity电阻系数（r（ij）），并且对于类离子相互作用获得的负值与伪离子液体描述一致，即混合物确实是非常弱的电解质。混合物的结构和旋转动力学也进行了研究，使用高能X射线全散射实验，分子动力学模拟，和介电弛豫光谱。基于活化能的比较和众所周知的线性自由能关系之间的动力学和热力学的自质子分解，我们提出了[C（1）HIm] [乙酸]的Grotthus型质子传导机制，涉及快速AcOH/AcO-旋转作为一个决定性的步骤。
In a previous work, we have found that the pseudo-protic ionic liquid N-methylimidazolium acetate, [C(1)HIm] [OAc] or [Hmim] [OAc], mainly consists of the electrically neutral molecular species N-methylimidazole, C(1)Im, and acetic acid, AcOH, even though the mixture has significant ionic conductivity. This system was revisited by employing isotopic substitution Raman spectroscopy (ISRS) and pulsed field gradient (PFG) NMR self-diffusion measurements. The ISRS and PFG-NMR results obtained fully confirm our earlier findings. In particular, the self-diffusion coefficient of the hydroxyl hydrogen atom in AcOH is identical to that of the methyl hydrogen atoms within the experimental uncertainty, consistent with very little ionization. Therefore, a proton conduction mechanism similar to the Grotthuss mechanism for aqueous acid solutions is postulated to be responsible for the observed electrical conductivity. Laity resistance coefficients (r(ij)) are calculated from the transport properties, and the negative values obtained for the like-ion interactions are consistent with the pseudo-ionic liquid description, that is, the mixture is indeed a very weak electrolyte. The structure and rotational dynamics of the mixture were also investigated using high-energy X-ray total scattering experiments, molecular dynamics simulations, and dielectric relaxation spectroscopy. Based on a comparison of activation energies and the well-known linear free energy relationship between the kinetics and thermodynamics of autoprotolysis, we propose for [C(1)HIm] [OAc] a Grotthus-type proton conduction mechanism involving fast AcOH/AcO- rotation as a decisive step.