Hydrogen Bond Donicity and Electron Pair Acceptance

The Gutmann-Mayer acceptor number scale AN [49] uses the NMR chemical shift 5 of the atom of triethylphosphine oxide as the electron-pair donor probe in dilute solution in the solvent to be smdied relative to the shift in n-hexane. The AN values are normalized to make AN=2.348(5/ppm), the 5 values being corrected for the diamagnetic susceptibility / of the solvent The AA scale does include solvent polarity effects besides its ability to donate hydrogen bonds, as is seen in its being proportional to the Ej values AN = 56.SE [37]. In fact, both scales are sensitive to both the solvent polarity and its acidity. The acceptor numbers of solvents are Usted in Table 3.9. 

The Kamlet-Taft a scale [48], on the other hand, was designed to show the net hydrogen bond donation or electron pair acceptance ability of a solvent, being 

For other probes, namely the AN and Ej scales, the values need corrections for the solvent polarity  

The a values obtained in this manner have an uncertainty of 0.08 and are listed in Table 3.9 adopted from [3]. Water has a large value of a = 1.17, but certain phenols and halogen-substituted alkanols and carboxylic acids are considerably more acidic hexafluoro-i-propanol has a = 1.96, dichloroacetic add has a = 2.24, and trifluoro-methanesulfonic acid has AN = 131.7, much higher than the value for water, 54.8, but these are rarely used as solvents for electrolytes and therefore are not dealt with further here. 

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