Rotational correlation times, diffusion coefficients and quadrupolar peaks of the protic ionic liquid ethylammonium nitrate by means of 1H fast field cycling NMR relaxometry
Rotational correlation times, diffusion coefficients and quadrupolar peaks of the protic ionic liquid ethylammonium nitrate by means of 1H fast field cycling NMR relaxometry
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DOI：
10.1016/j.molliq.2020.114983
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发表时间：
2021-01-15
影响因子：
6
通讯作者：
Ludwig, Ralf
中科院分区：
文献类型：
--
作者：
Overbeck, Viviane;Appelhagen, Andreas;Ludwig, Ralf
质子离子液体（PIL）硝酸乙胺（EAN）的特点是阳离子的N-H键与阴离子的氧原子之间存在氢键。EAN的三维氢键网络与水的非常相似。虽然这第一种离子液体已经被发现了100多年，但仍然缺乏关于旋转和平动动力学的信息，以及两者之间的关系。为此，我们采用快速场循环（FFC）核磁共振弛豫仪测量了EAN的H-1核磁弛豫色散曲线（NMRD）。我们能够同时分析在248 ~ 333 K的大温度范围内测量到的H-1自旋晶格弛豫率R-1。应用著名的Bloembergen-Purcell-Pound （BPP）方法，我们将总弛缓速率分解为分子内和分子间的贡献，描述了乙基铵阳离子的旋转和动动动力学。在这里，我们报告了旋转相关时间，tau(R)和平移扩散系数，D-T，它们都是从总H-1自旋晶格弛豫速率作为温度和频率的函数推导出来的。自扩散系数D-T也由低频范围内的总弛豫速率确定，其中只有分子间弛豫贡献与频率有关。然后，D-T由自旋-晶格弛豫速率R-1的线性拟合斜率作为频率平方根的函数得到。ν。两种方法得到的扩散系数与脉冲场梯度核磁共振（PFG）测量的自扩散系数吻合良好。将分子内H-1弛豫贡献得到的旋转相关时间tau(R)与高场核磁共振氘子弛豫实验、介电光谱（DS）和飞秒红外（fs-IR）光谱得到的相关时间进行了比较。对于EAN，我们还观察到H-1自旋晶格弛豫率R-1在高频处的局部增强。这种所谓的四极弛豫增强（QRE）是由H-1自旋的塞曼跃迁能和N-14四极核两能级的能量差的干涉引起的。QRE通常用于固体，但很少用于液体，如这里所观察到的EAN。总的来说，我们表明FFC弛豫测量同时提供了旋转相关时间、平移扩散系数和四极弛豫增强。(C) 2020 Elsevier B.V.版权所有
The protic ionic liquid (PIL) ethylammonium nitrate (EAN) is characterized by hydrogen bonding between the N-H bonds of the cations and the oxygen atoms of the anions. The three-dimensional H-bond network of EAN very much resembles that of water. Although this first ionic liquid is known for more than 100 years, there is still lack of information about the rotational and translational dynamics, and how both are related to each other. For that purpose, we measured the H-1 nuclear magnetic relaxation dispersion curves (NMRD) of EAN by means of Fast Field Cycling (FFC) NMR relaxometry. We were able to analyze the measured H-1 spinlattice relaxation rates R-1 covering a large temperature range between 248 and 333 K simultaneously. Applying the well-known Bloembergen-Purcell-Pound (BPP) approach, we decomposed the total relaxation rates into intramolecular and intermolecular contributions, describing the rotational and translational dynamics of the ethylammonium cation. Here, we report rotational correlation times, tau(R) and translational diffusion coefficients, D-T, both derived from the total H-1 spin-lattice relaxation rates as a function of temperature and frequency. Self-diffusion coefficients, D-T were also determined from the total relaxation rates in the low frequency range, wherein only the intermolecular relaxation contribution is frequency dependent. Then, D-T results from the slope of a linear fit of the spin-lattice relaxation rate R-1 as a function of the square root of frequency. nu. The diffusion coefficients obtained from both methods are in good agreement with self-diffusion coefficients measured by pulsed-field-gradient (PFG) NMR. The rotational correlation times, tau(R), derived from the intramolecular H-1 relaxation contribution were compared to correlation times obtained from high-field NMR deuteron relaxation experiments, dielectric spectroscopy (DS) and femto-second-infrared (fs-IR) spectroscopy. For EAN, we also observed a local enhancement of the H-1 spin-lattice relaxation rates R-1 at high frequencies. This so-called quadrupole relaxation enhancement (QRE) results from the interference of the Zeeman transition energy of the H-1 spins and the energy difference of two levels of the quadrupole N-14 nuclei. QRE is usually known for solids, but rarely for the liquid state as observed here for EAN. Overall, we show that FFC relaxometry provides access to rotational correlation times, translational diffusion coefficients and quadrupole relaxation enhancement at the same time. (C) 2020 Elsevier B.V. All rights reserved.