A unified scale for understanding and predicting non-specific solvent polarity: a dynamic cavity model

Data, which have provided the basis for several different solvation scales, are combined to produce a universal scale of solvent polarity. Over three hundred spectral shifts for 30 probe molecules and 31 solvents are fitted to an equation of the form Δχ=S′P+W. Electronic transitions, NMR chemical shifts and EPR hyperfine coupling constants all fit to the same solvent S′ values. The data fit a wide variety of probe shapes and sizes leading to the proposal of a dynamic cavity model. The success of the approach is attributed to the exclusion from the fit of data involving (i) concentrated solutions of polar probes in non-polar solvents, (ii) specific donor–acceptor interactions and (iii) polar solvents that exist as rotamers which solvate to different extents. A generalized scale of non-specific solvating ability is offered for use in the analysis of solvent effects. Deviations of results from those predicted by this scale signal unusual behaviour and experiments can be designed to identify the cause. Compared to the current practice of fitting results to one of the several ‘solvation’ scales now available, a scale of non-specific solvation can provide the basis for a more detailed understanding of reactivity and spectroscopy in solution.

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