Determination of a Solvent Hydrogen-Bond Acidity Scale by Means of the Solvatochromism of Pyridiniurn-N-phenolate Betaine Dye 30 and PCM-TD-DFT Calculations

Empirical parameters of solvents describing their hydrogen-bond (HB) acidity (e.g., the Kamlet-Taft alpha parameter) are often difficult to determine for new solvents because they are not directly related to a single definition process. Here, we propose a simple method based on one probe, the betaine dye 30, and one reference process, the solvatochromism of this dye, measured by its first electronic transition energy, E-T(30). These E-T(30) values are calculated within the time-dependent density functional theory framework, using a polarizable continuum solvent model (PCM). The part of E-T(30) values that is not included in the PCM calculation is taken as the FIB component of the measured E-T(30) values, allowing us to deduce a solvent HB acidity parameter alpha(1). The validity of this simple model is assessed by good linear correlations between alpha(1) and a variety of solute properties mainly depending on the solvent's HB acidity. The quality of fit observed with alpha(1) is at least comparable with that obtained by previous solvent HB acidity scales. The simplicity of our method is illustrated by the determination of alpha(1) and of its companion, the electrostatic solvent parameter ES, for some new green solvents derived from glycerol.