Elola M Dolores, Ladanyi Branka M, Scodinu Alessandra, Loughnane Brian J, Fourkas John T
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, USA.
J Phys Chem B. 2005 Dec 22;109(50):24085-99. doi: 10.1021/jp054733n.
In this work we have studied the relaxation dynamics of the many-body polarizability anisotropy in liquid mixtures of benzene (Bz) and hexafluorobenzene (Hf) at room temperature by femtosecond optical heterodyne-detected Raman-induced Kerr effect spectroscopy (OHD-RIKES) experiments and molecular dynamics (MD) simulations. The computed polarizability response arising from intermolecular interactions was included using the first-order dipole-induced-dipole model with the molecular polarizability distributed over the carbon sites of each molecule. We found good qualitative agreement between experiments and simulations in the features exhibited by the nuclear response function R(t) for pure liquids and mixtures. The long-time diffusive decay of R(t) was observed to vary substantially with composition, slowing down noticeably with dilution of each of the species as compared with that in the corresponding pure liquids. MD simulation shows that the effect on R(t) is due to the formation of strong and localized intermolecular association between Bz and Hf species that hinder the rotational diffusive dynamics. The formation of these Bz-Hf complexes in the liquid mixtures also modifies the rotational diffusive dynamics of the component species in such a way that cannot be explained solely in terms of a viscosity effect. Even though the computed orientational diffusive relaxation times associated with Bz and Hf are larger by a factor of approximately 2 than those from experiments, we found similar trends in experiments and simulations for these characteristic times as a function of composition. Namely, the collective and single-molecule orientational correlation times associated with Bz are observed to grow monotonically with the dilution of Bz, while those corresponding to Hf species exhibit a maximum at the equimolar composition. We attribute the quantitative discrepancy between experiments and simulations to the use of the Williams potential, which seems to overestimate the intermolecular interactions and thus predicts not only a slower translational dynamics but also a slower rotational diffusion dynamics than in real fluids.
在这项工作中，我们通过飞秒光学外差检测拉曼诱导克尔效应光谱（OHD-RIKES）实验和分子动力学（MD）模拟，研究了室温下苯（Bz）和六氟苯（Hf）液体混合物中多体极化率各向异性的弛豫动力学。使用一阶偶极诱导偶极模型，将分子极化率分布在每个分子的碳位点上，从而纳入了由分子间相互作用产生的计算极化率响应。我们发现，在纯液体和混合物的核响应函数R(t)所展现的特征方面，实验与模拟之间存在良好的定性一致性。观察到R(t)的长时间扩散衰减随组成有很大变化，与相应纯液体相比，每种物质稀释时其衰减明显减慢。MD模拟表明，对R(t)的影响是由于Bz和Hf物种之间形成了强烈且局部的分子间缔合，这阻碍了旋转扩散动力学。液体混合物中这些Bz-Hf络合物的形成还以一种不能仅用粘度效应来解释的方式改变了组分物种的旋转扩散动力学。尽管计算得到的与Bz和Hf相关的取向扩散弛豫时间比实验值大约大2倍，但我们发现这些特征时间在实验和模拟中随组成的变化趋势相似。也就是说，观察到与Bz相关的集体和单分子取向相关时间随Bz稀释而单调增加，而与Hf物种对应的时间在等摩尔组成时出现最大值。我们将实验与模拟之间的定量差异归因于威廉姆斯势的使用，该势似乎高估了分子间相互作用，因此不仅预测平移动力学比实际流体慢，而且旋转扩散动力学也比实际流体慢。
