烷基柔性对咪唑类离子液体熔点的影响
Effect of Alkyl-group Flexibility on the Melting Point of Imidazolium-based Ionic Liquids
摘要：
室温离子液体的低熔点通常用大体积、低对称性和柔性离子的存在来解释，前两个因素与晶格能有关，而熵效应归因于后者。通过分子动力学模拟，测定了1-乙基-3-甲基咪唑六氟磷酸盐和1-癸基-3-甲基咪唑六氟磷酸盐的熔点，并通过抑制固相和液相中二面角的旋转，明确计算了分子柔性对熔点的影响.在环和烷基链之间的键的旋转的灵活性影响的相对顺序的阴离子周围的阳离子和结果在熔化的焓和熵的实质性影响。对于烷基的其他二面角，贡献主要是熵，并发现了交替的行为，一些二面角的灵活性对熔点的影响可以忽略不计，而另一些二面角的灵活性可以导致熔点的差异高达20 K。这种交替的行为是合理的，在晶体中的构象缺陷的不同概率。
英文摘要：
The low melting point of room temperature ionic liquids is usually explained in terms of the presence of bulky, low symmetry and flexible ions, with the first two factors related to the lattice energy while an entropic effect is attributed to the latter. By means of molecular dynamics simulations, the melting points of 1-ethyl-3-methylimidazolium hexafluorophosphate and 1-decyl-3-methyl-imidazolium hexafluorophosphate were determined and the effect of the molecular flexibility over the melting point was explicitly computed by restraining the rotation of dihedral angles in both the solid and the liquid phase. The rotational flexibility over the bond between the ring and the alkyl chain affects the relative ordering of the anions around the cations and results in substantial effects over both the enthalpy and the entropy of melting. For the other dihedral angles of the alkyl group, the contributions are predominantly entropic and an alternating behavior was found. The flexibility of some dihedral angles has negligible effects on the melting point, while others can lead to differences in the melting point as large as 20 K. This alternating behavior is rationalized by the different probabilities of conformation defects in the crystal.
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