Solvent Basicity, A Study of Kamlet–Taft β and Gutmann DN Values Using Computationally Derived Molecular Properties

Abstract

Solvent basicity is recognized as playing a major role in solvation and is included, through empirical basicity parameters, in linear free energy relationships that account for the effects of changes in solvent on chemical reactions. It is reasonable to postulate that the basicity of a solvent molecule reflects some combination of its molecular properties. In the present study, density functional calculations using the B3LYP functional, and Hartree–Fock calculations have been used to calculate the partial atomic charges (using the Hirshfeld and CM5 models), orbital energies, polarizabilities, dipole moments and quadrupolar amplitudes for over one hundred molecules for which there are experimental values for two basicity parameters, Kamlet and Taft’s hydrogen bond acceptor strength, β, and Gutmann’s donor number, DN, a measure of Lewis basicity. Regression of the experimental β and DN values against molecular descriptors reflecting the above molecular properties yields a remarkably consistent picture. For both parameters the values for alcohols and amines lie systematically off of the regression lines through the remaining compounds, which include alkanes, aromatics, halogenated alkanes and aromatics, esters, carbonates, carboxylic acids, ketones, ethers, nitriles, phosphates, sulfides and sulfates. Independent of the calculation method or method of estimating the partial atomic charges, both experimental β and DN are essentially determined by two molecular properties: the charge on the most negative atom of the molecule and the molecular orbital from which charge donation would occur. The regression results using any of the fours sets of descriptors (reflecting the two calculation methods and two methods of charge estimation) are remarkably similar for β and DN supporting the view that these are measures of the same “basicity”.