James Franck Institute, The University of Chicago, Chicago, Illinois 60637, United States.
Department of Chemistry, The University of Chicago, Chicago, Illinois 60637, United States.
J Phys Chem B. 2022 Jan 13;126(1):278-291. doi: 10.1021/acs.jpcb.1c09572. Epub 2021 Dec 28.
Acetonitrile has emerged as a solvent candidate for novel electrolyte formulations in metal-ion batteries and supercapacitors. It features a bright local C≡N stretch vibrational mode whose infrared (IR) signature is sensitive to battery-relevant cations (Li, Mg, Zn, Ca) both in pure form and in the presence of water admixture across a full possible range of concentrations from the dilute to the superconcentrated regime. Stationary and time-resolved IR spectroscopy thus emerges as a natural tool to study site-specific intermolecular interactions from the solvent perspective without introducing an extrinsic probe that perturbs solution morphology and may not represent the intrinsic dynamics in these electrolytes. The metal-coordinated acetonitrile, water-separated metal-acetonitrile pair, and free solvent each have a distinct vibrational signature that allows their unambiguous differentiation. The IR band frequency of the metal-coordinated acetonitrile depends on the ion charge density. To study the ion transport dynamics, it is necessary to differentiate energy-transfer processes from structural interconversions in these electrolytes. Isotope labeling the solvent is a necessary prerequisite to separate these processes. We discuss the design principles and choice of the CDCN label and characterize its vibrational spectroscopy in these electrolytes. The Fermi resonance between C≡N and C-D stretches complicates the spectral response but does not prevent its effective utilization. Time-resolved two-dimensional (2D) IR spectroscopy can be performed on a mixture of acetonitrile isotopologues and much can be learned about the structural dynamics of various species in these formulations.
乙腈已成为金属离子电池和超级电容器新型电解质配方的候选溶剂。它具有明亮的局部 C≡N 伸缩振动模式，其红外（IR）特征对纯形式以及在水混合物存在下的相关阳离子（Li、Mg、Zn、Ca）均敏感，跨越从稀溶液到超浓溶液的整个可能浓度范围。因此，固定和时间分辨红外光谱成为一种从溶剂角度研究特定于位置的分子间相互作用的自然工具，而无需引入会干扰溶液形态且可能无法代表这些电解质内在动力学的外在探针。配位乙腈、水分离的金属-乙腈对和游离溶剂各自具有独特的振动特征，可明确区分。配位乙腈的 IR 带频率取决于离子电荷密度。为了研究离子输运动力学，有必要区分这些电解质中的能量转移过程和结构互变。对溶剂进行同位素标记是分离这些过程的必要前提。我们讨论了 CDCN 标记的设计原则和选择，并在这些电解质中对其振动光谱进行了表征。C≡N 和 C-D 伸缩的费米共振使光谱响应复杂化，但不会阻止其有效利用。可在乙腈同位素混合物上进行二维（2D）IR 光谱的时间分辨实验，并可了解这些配方中各种物质的结构动力学。
