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

The strength of CH-aryl interactions (Delta 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 Delta 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 Delta 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 [Delta G = -0.24 + 0.23 alpha - 0.68 beta - 0.1 pi* + 0.09 delta]-expresses Delta G as a function of Kamlet-Taft solvent parameters-revealed that specific solvent effects (alpha and beta) are mainly responsible for "tipping" the molecular balance in favour of one conformer over the other, where a represents a solvents' hydrogen-bond acidity and beta represents a solvents' hydrogen-bond basicity. Furthermore, using extrapolated data (alpha and beta) and the known pi* value for the gas phase, the LSER equation predicted DG in the gas phase to be -0.31 kcal mol(-1), which agrees with -.35 kcal mol(-1) estimated from DFT-D calculations.