Optical basicity scales in protic solvents: water, hydrogen fluoride, ammonia and their mixtures

In an inorganic material, a measure of the Lewis basicity of a solvent is commonly provided by the optical basicity, i.e. the ability of the solvent molecules to donate their electrons to an acidic species. This quantity is known to vary with the polarizability of the Lewis base. In protic solvents, the Lewis definition of basicity is barely used; it is replaced by the more purpose-built Brønsted-Lowry scale, and its generalized variant proposed by Hammett. In this study, individual molecular polarizabilities were computed from first-principles for a series of protic solvents: pure water, hydrogen fluoride, ammonia and their mixtures. From these calculations optical basicity scales were set up for each Lewis base. It was shown that these scales correlate with the Hammett acidity. It is therefore possible to build a common optical basicity scale, in which any material (protic solvents, inorganic materials) can unambiguously be classified.