Pressure and temperature dependence of the hydrogen bonding in supercritical ethanol: a computer simulation study.
Pressure and temperature dependence of the hydrogen bonding in supercritical ethanol: a computer simulation study.
复制标题
超临界乙醇中氢键的压力和温度依赖性：计算机模拟研究。
DOI：
10.1021/jp051688a
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发表时间：
2005
期刊：
影响因子：
--
通讯作者：
J. Samios
中科院分区：
文献类型：
--
作者：
D. Dellis;M. Chalaris;J. Samios
为了更深入地了解超临界乙醇（scEtOH）中的“氢键”，我们对该流体在很宽的温度和压力范围内进行了NVT分子动力学模拟。在SC条件下对流体进行了热力学和光谱（核磁共振、红外、拉曼、介电）数据的研究。计算了各位点对分布函数，得到了它们与温度和压力的依赖关系。研究发现，在热力学条件下，scEtOH保持高度结构化。此外，H-H、O-O和H-O的首峰特征行为表明，氢键仍然存在于scEtOH中。分析还着重于键单位向量O-H， C-O和分子的永久偶极矩以及样品的总偶极矩的重定向动力学。结合流体中的“氢键”和实验结果，讨论了相应的勒让德时间相关函数。具体来说，O-H动力学的行为表现出系统中分子的众所周知的结合性质。对氢键进行了进一步的分析，得到了聚集度（每个分子中氢键的平均数目），并与核磁共振化学位移研究的结果进行了比较。并与红外光谱和拉曼振动光谱结果进行了比较。液体和乙醇分子的百分比分析fi， i = 0,1,2,3，…得到了每个分子的氢键。结果表明，存在主要由两个分子组成的小的线性链低聚物，而样品中三体低聚物的数量，特别是四体低聚物的数量相对较少。
As a step toward deeper insight on the "hydrogen bonding" in supercritical ethanol (scEtOH), we carried out NVT molecular dynamics simulations of the fluid over a wide range of temperatures and pressures. The fluid was studied at SC conditions for which thermodynamic and spectroscopic (NMR, infrared, Raman, dielectric) data are available. The various site-site pair distribution functions (pdf's) were calculated, and their temperature and pressure dependence was obtained. It was found that over the thermodynamic conditions investigated here, scEtOH remains highly structured. Moreover, the characteristic behavior of the first peaks in H-H, O-O, and H-O pdf's reveals that hydrogen bonds still exist in scEtOH. The analysis focuses also on the reorientational dynamics of the bond unit vectors O-H, C-O, and of the permanent dipole moment of the molecules as well as the total dipole moment of the sample. The corresponding Legendre time correlation functions were discussed in connection to the "hydrogen bonding" in the fluid and in the context of experimental results. Specifically, the behavior of the O-H dynamics exhibits the well-known associative nature of the molecules in the system. A further analysis of the hydrogen bonds was carried out, and the degree of aggregation (average number of H-bonds per molecule) was obtained and compared with results from NMR chemical shift studies. Also the estimated monomer and free O-H groups in the fluid were compared with results from IR and Raman vibrational spectroscopy. The percentage analysis fi of the liquid and scEtOH molecules, with i = 0, 1, 2, 3, ... hydrogen bonds per molecule, has been obtained. The results show the existence of small, linear-chain oligomers formed mainly by two molecules, whereas the number of the three body oligomers, and specifically that of four body oligomers in the sample, is relatively small.