Fachbereich Physik, Freie Universität Berlin, 14195 Berlin, Germany.
Institut für Physikalische und Theoretische Chemie, Universität Regensburg, D-93040 Regensburg, Germany.
J Chem Phys. 2018 Jun 14;148(22):222812. doi: 10.1063/1.5000385.
We compare the dielectric spectra of aqueous MgSO and NaSO solutions calculated from classical molecular dynamics simulations with experimental data, using an optimized thermodynamically consistent sulfate force field. Both the concentration-dependent shift of the static dielectric constant and the spectral shape match the experimental results very well for NaSO solutions. For MgSO solutions, the simulations qualitatively reproduce the experimental observation of a slow mode, the origin of which we trace back to the ion-pair relaxation contribution via spectral decomposition. The radial distribution functions show that Mg and SO ions form extensive water-separated-and thus strongly dipolar-ion pairs, the orientational relaxation of which provides a simple physical explanation for the prominent slow dielectric mode in MgSO solutions. Remarkably, the Mg-SO ion-pair relaxation extends all the way into the THz range, which we rationalize by the vibrational relaxation of tightly bound water-separated ion pairs. Thus, the relaxation of divalent ion pairs can give rise to widely separated orientational and vibrational spectroscopic features.
我们将从经典分子动力学模拟中计算出的含镁硫酸盐和硫酸钠水溶液的介电谱与实验数据进行比较，使用了优化的热力学一致的硫酸盐力场。硫酸钠溶液的静态介电常数的浓度依赖性位移和光谱形状都与实验结果非常吻合。对于硫酸镁溶液，模拟定性地再现了实验观察到的慢模，其起源可以通过光谱分解追溯到离子对弛豫的贡献。径向分布函数表明，镁和硫酸根离子形成广泛的水分离且因此具有强烈偶极离子对，其取向弛豫为硫酸镁溶液中突出的慢介电模式提供了简单的物理解释。值得注意的是，Mg-SO 离子对弛豫一直延伸到太赫兹范围，我们通过紧密结合的水分离离子对的振动弛豫来解释这一点。因此，二价离子对的弛豫可以产生广泛分离的取向和振动光谱特征。
