A Generalized Solvent Acidity Scale: The Solvatochromism of o-tert-Butylstilbazolium Betaine Dye and Its Homomorph o,o′-Di-tert-butylstilbazolium Betaine Dye†

A total of 121 organic solvents were studied and classified by means of the solvatochromic comparison method of Kamlet and Taft, to give a solvent acidity scale for hydrogen-bond donor solvents based on the parameter SA. This parameter can readily be evaluated from the visible spectrum of a suitable basic probe (o-tert-butylstilbazolium betaine dye, TBSB) and its non-basic homomorph (o,o′-di-tert-butylstilbazolium betaine dye, DTBSB). The proposed scale has many advantages when compared to the more widely used acidity scales such as that based on Gutmann's acceptor number (AN) and the Taft-Kamlet α scale. Whilst data for the proposed scale are derived from electronic transitions only, they do provide an accurate description for solvent acidity effects in spectroscopy and other chemical areas, including thermodynamics and kinetics.

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