Quantitative model for rationalizing solvent effect in noncovalent CH-Aryl interactions

The strength of CH-aryl interactions (Δ G ) in 14 solvents was determined via the conformational analysis of a molecular torsion balance. The molecular balance adopted folded and unfolded conformers in which the ratio of the conformers in solution provided a quantitative measure of Δ G as a function of solvation. While a single empirical solvent parameter based on solvent polarity failed to explain solvent effect in the molecular balance, it is shown that these Δ G values can be correlated through a multiparameter linear solvation energy relationship (LSER) using the equation introduced by Kamlet and Taft. The resulting LSER equation [Δ G = −0.24 + 0.23 α − 0.68 β − 0.1 π * + 0.09 δ ]-expresses Δ G as a function of Kamlet-Taft solvent parameters-revealed that specific solvent effects ( α and β ) are mainly responsible for "tipping" the molecular balance in favour of one conformer over the other, where α represents a solvents' hydrogen-bond acidity and β represents a solvents' hydrogen-bond basicity. Furthermore, using extrapolated data ( α and β ) and the known π * value for the gas phase, the LSER equation predicted Δ G in the gas phase to be −0.31 kcal mol −1 , which agrees with −0.35 kcal mol −1 estimated from DFT-D calculations. Establishing a linear relationship between CH-aryl interaction energies and the properties of the solvating media.