Chen Lihua, Bryantsev Vyacheslav S
Department of Materials Science and Engineering, University of Connecticut, Storrs, CT 06269, USA and Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.
Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA.
Phys Chem Chem Phys. 2017 Feb 1;19(5):4114-4124. doi: 10.1039/c6cp08403f.
Accurate prediction of melting points of ILs is important both from the fundamental point of view and from the practical perspective for screening ILs with low melting points and broadening their utilization in a wider temperature range. In this work, we present an ab initio approach to calculate melting points of ILs with known crystal structures and illustrate its application for a series of 11 ILs containing imidazolium/pyrrolidinium cations and halide/polyatomic fluoro-containing anions. The melting point is determined as a temperature at which the Gibbs free energy of fusion is zero. The Gibbs free energy of fusion can be expressed through the use of the Born-Fajans-Haber cycle via the lattice free energy of forming a solid IL from gaseous phase ions and the sum of the solvation free energies of ions comprising IL. Dispersion-corrected density functional theory (DFT) involving (semi)local (PBE-D3) and hybrid exchange-correlation (HSE06-D3) functionals is applied to estimate the lattice enthalpy, entropy, and free energy. The ions solvation free energies are calculated with the SMD-generic-IL solvation model at the M06-2X/6-31+G(d) level of theory under standard conditions. The melting points of ILs computed with the HSE06-D3 functional are in good agreement with the experimental data, with a mean absolute error of 30.5 K and a mean relative error of 8.5%. The model is capable of accurately reproducing the trends in melting points upon variation of alkyl substituents in organic cations and replacement one anion by another. The results verify that the lattice energies of ILs containing polyatomic fluoro-containing anions can be approximated reasonably well using the volume-based thermodynamic approach. However, there is no correlation of the computed lattice energies with molecular volume for ILs containing halide anions. Moreover, entropies of solid ILs follow two different linear relationships with molecular volume for halides and polyatomic fluoro-containing anions. Continuous progress in predicting crystal structures of organic salts with halide anions will be a key factor for successful prediction of melting points with no prior knowledge of the crystal structure.
从基础研究角度以及从实际应用角度来看，准确预测离子液体（ILs）的熔点都非常重要，这有助于筛选低熔点离子液体，并拓宽其在更宽温度范围内的应用。在本工作中，我们提出了一种从头算方法来计算具有已知晶体结构的离子液体的熔点，并展示了其在一系列包含咪唑鎓/吡咯烷鎓阳离子以及卤化物/含氟多原子阴离子的11种离子液体中的应用。熔点被定义为熔融吉布斯自由能为零的温度。熔融吉布斯自由能可以通过玻恩 - 法扬斯 - 哈伯循环，利用从气相离子形成固态离子液体的晶格自由能以及构成离子液体的离子的溶剂化自由能之和来表示。采用包含（半）局域（PBE - D3）和杂化交换相关（HSE06 - D3）泛函的色散校正密度泛函理论（DFT）来估算晶格焓、熵和自由能。在标准条件下，使用SMD - 通用 - IL溶剂化模型在M06 - 2X/6 - 31 + G(d)理论水平下计算离子的溶剂化自由能。用HSE06 - D3泛函计算得到的离子液体熔点与实验数据吻合良好，平均绝对误差为30.5 K，平均相对误差为8.5%。该模型能够准确再现有机阳离子中烷基取代基变化以及一种阴离子被另一种阴离子取代时熔点的变化趋势。结果证实，对于含氟多原子阴离子的离子液体，使用基于体积的热力学方法可以较好地近似其晶格能。然而，对于含卤化物阴离子的离子液体，计算得到的晶格能与分子体积之间没有相关性。此外，固态离子液体的熵对于卤化物和含氟多原子阴离子与分子体积遵循两种不同的线性关系。在不预先了解晶体结构的情况下成功预测熔点，预测含卤化物阴离子的有机盐晶体结构的持续进展将是一个关键因素。
