Theoretical, Semiempirical, and Experimental Solvatochromic Comparison Methods for the Construction of the α1 Scale of Hydrogen-Bond Donation of Solvents

Today, the hydrogen bonding donation (HBD) ability parameter of new solvents, α, is generally determined either by the Kamlet–Taft solvatochromic comparison of two probes, Reichardt betaine dye B(30) and 4-nitroanisole, or by the measurement of a single probe (e.g., solvatochromism of an iron coordination complex). This work highlights the shortcomings of these probes and recommends three replacement methods: (a) the theoretical comparison of the experimental and PCM-TD-DFT calculated transition energies E T(30) of B(30), (b) the semiempirical comparison of the experimental and McRae calculated E T(30), and, (c) for ionic liquids, the experimental comparison of E T(30) and E T(33) lying on the lower basicity of the betaine dye B(33) compared to B(30). These methods yield a new HBD parameter, α1, for 101 molecular solvents and 30 ionic liquids. The novelty is emblematic for water, with α1 = 1.54 instead of α (Kamlet–Taft) = 1.17. The solvent parameter α1 is not equivalent to the solute hydrogen-bond acidity parameter α2 H , partly because of the self-association of HBD solvents.