The chemical hardness of a solvent can play a decisive role in solubility and reactivity in solution. Several empirical scales quantifying solvent softness have been proposed. We explore whether computed properties of solvent molecules can reproduce these experimental scales. Our “orbital overlap distance” quantifying the size of orbitals at a molecule’s surface effectively reproduces the Marcus μ-scale of solvent softness. The orbital overlap distance predicts that the surface of chemically hard solvent molecules is dominated by compact orbitals possessing a small orbital overlap distance. In contrast, the surface of chemically soft solvent molecules has a larger contribution from diffuse orbitals and a larger orbital overlap distance. Other conceptual density functional theory descriptors, including the global hardness and electronegativity, can also reproduce the Marcus scale. We further introduce a “solvent versatility” RMSD Dsurf scale quantifying variations in the surface orbital overlap distance. “Good” solvents such as DMSO, which combine chemically “hard” and “soft” sites within a single molecule, possess a large RMSD Dsurf. We conclude by applying this approach to predict the Marcus μ-parameters for widely-used ionic liquids and ionic liquid–cosolvent systems.

中文翻译：

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使用概念密度泛函理论和轨道重叠距离扩展溶剂软度的 Marcus μ 尺度：离子液体的方法和应用

溶剂的化学硬度对溶液中的溶解度和反应性起着决定性的作用。已经提出了几种量化溶剂柔软度的经验尺度。我们探索溶剂分子的计算特性是否可以重现这些实验规模。我们的“轨道重叠距离”量化了分子表面轨道的大小，有效地再现了溶剂柔软度的 Marcus μ 尺度。轨道重叠距离预测化学硬溶剂分子的表面由具有小轨道重叠距离的致密轨道支配。相比之下，化学软溶剂分子的表面具有更大的扩散轨道贡献和更大的轨道重叠距离。其他概念密度泛函理论描述符，包括全局硬度和电负性，也可以重现马库斯量表。我们进一步引入了“溶剂多功能性”RMSD Dsurf 尺度，量化了表面轨道重叠距离的变化。“好”溶剂，如 DMSO，在单个分子内结合了化学“硬”和“软”位点，具有大的 RMSD Dsurf。最后，我们应用这种方法来预测广泛使用的离子液体和离子液体-共溶剂系统的 Marcus μ 参数。