Institute for Computational Molecular Science and Department of Chemistry, Temple University, Philadelphia, Pennsylvania 19122, USA.
J Chem Phys. 2012 Feb 21;136(7):074508. doi: 10.1063/1.3685100.
The rotational dynamics of benzene and water in the ionic liquid (IL) 1-butyl-3-methylimidazolium chloride are studied using molecular dynamics (MD) simulation and NMR T(1) measurements. MD trajectories based on an effective potential are used to calculate the (2)H NMR relaxation time, T(1) via Fourier transform of the relevant rotational time correlation function, C(2R)(t). To compensate for the lack of polarization in the standard fixed-charge modeling of the IL, an effective ionic charge, which is smaller than the elementary charge is employed. The simulation results are in closest agreement with NMR experiments with respect to the temperature and Larmor frequency dependencies of T(1) when an effective charge of ±0.5e is used for the anion and the cation, respectively. The computed C(2R)(t) of both solutes shows a bi-modal nature, comprised of an initial non-diffusive ps relaxation plus a long-time ns tail extending to the diffusive regime. Due to the latter component, the solute dynamics is not under the motional narrowing condition with respect to the prevalent Larmor frequency. It is shown that the diffusive tail of the C(2R)(t) is most important to understand frequency and temperature dependencies of T(1) in ILs. On the other hand, the effect of the initial ps relaxation is an increase of T(1) by a constant factor. This is equivalent to an "effective" reduction of the quadrupolar coupling constant (QCC). Thus, in the NMR T(1) analysis, the rotational time correlation function can be modeled analytically in the form of aexp (-t/τ) (Lipari-Szabo model), where the constant a, the Lipari-Szabo factor, contains the integrated contribution of the short-time relaxation and τ represents the relaxation time of the exponential (diffusive) tail. The Debye model is a special case of the Lipari-Szabo model with a = 1, and turns out to be inappropriate to represent benzene and water dynamics in ILs since a is as small as 0.1. The use of the Debye model would result in an underestimation of the QCC by a factor of 2-3 as a compensation for the neglect of the Lipari-Szabo factor.
采用分子动力学（MD）模拟和 NMR T(1)测量研究了苯和水在离子液体（IL）1-丁基-3-甲基咪唑氯化物中的旋转动力学。基于有效势能的 MD 轨迹用于通过相关旋转时间相关函数的傅里叶变换计算（2）H NMR 弛豫时间 T(1)。为了补偿 IL 中标准固定电荷建模中极化的缺乏，采用有效离子电荷，其小于基本电荷。当阴离子和阳离子的有效电荷分别为±0.5e 时，模拟结果与 NMR 实验在 T(1)的温度和拉莫尔频率依赖性方面最为吻合。两种溶质的计算 C(2R)(t)均表现出双模态性质，由初始非扩散 ps 弛豫和延伸至扩散区域的长时间 ns 尾巴组成。由于后一个组成部分，相对于普遍的拉莫尔频率，溶质动力学不受运动变窄条件的限制。结果表明，C(2R)(t)的扩散尾巴对于理解 IL 中 T(1)的频率和温度依赖性最为重要。另一方面，初始 ps 弛豫的影响是通过常数因子增加 T(1)。这相当于有效降低四极耦合常数（QCC）。因此，在 NMR T(1)分析中，旋转时间相关函数可以用形式为 aexp(-t/τ)（Lipari-Szabo 模型）的解析模型进行建模，其中常数 a，即 Lipari-Szabo 因子，包含短时间弛豫的积分贡献，τ代表指数（扩散）尾巴的弛豫时间。Debye 模型是 Lipari-Szabo 模型的特例，a=1，事实证明，它不适用于代表 IL 中的苯和水动力学，因为 a 小至 0.1。Debye 模型的使用会导致 QCC 低估 2-3 倍，作为对忽略 Lipari-Szabo 因子的补偿。
