Abstract
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.
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Supplementary file1 (DOCX 1036 kb) The following quantities are reported in the supplementary material. HOMO–LUMO surface plots of selective solvents, correlation between μ and 1/Gap, mean Dsurf and 1/Gap and μ and 1/Gap. Experimental μ values of solvent and their calculated values of mean Dsurf and RMSD Dsurf, with and without neutral regions. Calculated values of 1/Gap and 1/(I – A) for all solvents. Calculated values for ionic liquids. Experimental values of Ds and DN used in this study. Basis set dependence of Dsurf and RMSD Dsurf.
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Mehmood, A., Janesko, B.G. Extending the Marcus μ-Scale of Solvent Softness Using Conceptual Density Functional Theory and the Orbital Overlap Distance: Method and Application to Ionic Liquids. J Solution Chem 49, 614–628 (2020). https://doi.org/10.1007/s10953-020-00973-5
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DOI: https://doi.org/10.1007/s10953-020-00973-5