Johnson Clinton A, Parker Anthony W, Donaldson Paul M, Garrett-Roe Sean
Department of Chemistry, University of Pittsburgh, 219 Parkman Avenue, Pittsburgh, Pennsylvania 15260, USA.
Central Laser Facility, STFC, Rutherford Appleton Laboratory, Harwell Campus, Didcot, United Kingdom.
J Chem Phys. 2021 Apr 7;154(13):134502. doi: 10.1063/5.0044822.
Using ultrafast two-dimensional infrared spectroscopy (2D-IR), a vibrational probe (thiocyanate, SCN) was used to investigate the hydrogen bonding network of the protic ionic liquid ethyl-ammonium nitrate (EAN) in comparison to HO. The 2D-IR experiments were performed in both parallel (⟨ZZZZ⟩) and perpendicular (⟨ZZXX⟩) polarizations at room temperature. In EAN, the non-Gaussian lineshape in the FTIR spectrum of SCN suggests two sub-ensembles. Vibrational relaxation rates extracted from the 2D-IR spectra provide evidence of the dynamical differences between the two sub-ensembles. We support the interpretation of two sub-ensembles with response function simulations of two overlapping bands with different vibrational relaxation rates and, otherwise, similar dynamics. The measured rates for spectral diffusion depend on polarization, indicating reorientation-induced spectral diffusion (RISD). A model of restricted molecular rotation (wobbling in a cone) fully describes the observed spectral diffusion in EAN. In HO, both RISD and structural spectral diffusion contribute with similar timescales. This complete characterization of the dynamics at room temperature provides the basis for the temperature-dependent measurements in Paper II of this series.
使用超快二维红外光谱（2D-IR），一种振动探针（硫氰酸盐，SCN）被用于研究质子离子液体硝酸乙铵（EAN）的氢键网络，并与水（HO）进行比较。二维红外实验在室温下以平行（⟨ZZZZ⟩）和垂直（⟨ZZXX⟩）偏振进行。在EAN中，SCN的傅里叶变换红外光谱中的非高斯线形表明存在两个子集合。从二维红外光谱中提取的振动弛豫速率提供了两个子集合之间动力学差异的证据。我们通过对具有不同振动弛豫速率但动力学相似的两个重叠带进行响应函数模拟，支持了对两个子集合的解释。测量的光谱扩散速率取决于偏振，表明存在重取向诱导的光谱扩散（RISD）。受限分子旋转模型（在圆锥中摆动）完全描述了在EAN中观察到的光谱扩散。在HO中，RISD和结构光谱扩散在相似的时间尺度上都有贡献。这种在室温下对动力学的完整表征为该系列第二篇论文中的温度相关测量提供了基础。
