Theoretical, Semiempirical, and Experimental Solvatochromic Comparison Methods for the Construction of the α 1 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, α 1 , for 101 molecular solvents and 30 ionic liquids. The novelty is emblematic for water, with α 1 = 1.54 instead of α (Kamlet-Taft) = 1.17. The solvent parameter α 1 is not equivalent to the solute hydrogen-bond acidity parameter α 2 H , partly because of the self-association of HBD solvents.