The S scale of Drago

Drago developed a universal polarity scale (the S scale) from more than three hundred spectral data (electronic transitions, F and N chemical shifts and RSE coupling constants) for 30 solutes in 31 non-protic solvents from cyclohexane to propylene carbonate. He used the equation 

The S scale was constructed from carefully selected data. Thus, it excluded data for (a) all systems involving any contribution from donor-acceptor specific interactions (donor molecules where only measured in donor solvents and data for ttsolutes were excluded) (b) concentrated solutions of polar molecules in non-polar solvents (which might result in clustering) and (c) polar solvents occurring as rotamers (each rotamer would be solvated in a different way). 

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First of all, quantum calculations allow one to predict basicity scales in agreement with experiment provided that the calculations are performed on the preferred conformation of the isolated molecule. If this is not done, a given term within a consistent series may jump from one rank to another as a function of the conformation used for the calculations. The determinant role of preferred conformation on any property (barrier to internal rotation and inversion, dipole moment, first adiabatic ionization potential, acidity and basicity in the gas phase, energy of complexation to BF3, etc.) was clearly demonstrated. We further show the importance of the role of preferred conformation in explaining some of the anomalies in Drago s systematics. 

Drago s EJC analysis and Gutmann s donor/acceptor approach [53, 67] have been compared [200, 217, 218], Eq. (2-12) has been extended for specific and nonspecific interactions between solutes and polar solvents [219]. Various Lewis acidity and basicity scales for polar solvents have been examined and compared by Fawcett, who concluded that the donor/acceptor scales of Gutmann seem to be the most appropriate [341],... 

Conversely, the AN seale ean be viewed as a scale of hardness for adds since EtsPO is a hard reference base. Nevertheless, the merit of Gutmann s approach Kes in the fact that his scales provide both addic and basic parameters for amphoteric species, which is not the case with Drago s E and C classifications. 

Drago has proposed a "unified scale of solvent polarities" based on an extension of the E and C acid and base parameters discussed above. Each solvent is characterized by a parameter S and the change in some property, A%, of a probe system is given hy Ax = PS + W, where P and W are constants for Ae probe. If the probe is an acceptor (Lewis acid) in a donor solvent, then... 

It should be noted that the S values originally used by Drago" " to describe the polarity of 27 solvents are well correlated with the corresponding values on our dipolarity (SdP) scale (n = 27, r = 0.961). 

Drago, R. S. (1992). J. Org. Chem. 61, 6547. A paper dealing with the analysis of solvent polarity scales. 

The importanee of aeid-base interactions in various fields of ehemistry led to extensive research in the 1960s to obtain acid-base scales. This resulted in the Hard and Soft Aeids and Bases (HSAB) seales of Pearson [30], Drago s E and C eonstants [31], and Gutmann s donor and aeeeptor numbers [32]. Bolger and Miehaels [33] have used Bronsted acid base ehemistry to predict the adhesion of organie and inorganic species. 

Osegovic, J.P. and Drago, S.R. A solid acidity scale based on the P MAS NMR shift of chemisorbed triethylphopshine oxide. J. Catal 1999,182, 1-4. 

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