Cationic CHπ interactions as a function of solvation.

The energy (ΔG) of a cationic CHπ interaction was measured experimentally through the conformational studies of new molecular torsion balances using proton NMR spectroscopy. Each of the molecular balance adopted folded and unfolded conformations for which the ratio of the conformational equilibrium (i.e., folded/unfolded ratio) provided a quantitative measure of the ΔG as a function of solvation. An excellent linear solvation energy relationship between the ΔG values and the Hunter's solvent hydrogen-bond parameters (αs and ßs) revealed that electrostatic interaction is the physical origin of the observed conformational preferences in solution.

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

The strength of CH-aryl interactions (ΔG) 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 ΔG 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 ΔG values can be correlated through a multiparameter linear solvation energy relationship (LSER) using the equation introduced by Kamlet and Taft. The resulting LSER equation [ΔG = -0.24 + 0.23α - 0.68ß - 0.1π* + 0.09δ]-expresses ΔG as a function of Kamlet-Taft solvent parameters-revealed that specific solvent effects (α and ß) 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 ß represents a solvents' hydrogen-bond basicity. Furthermore, using extrapolated data (α and ß) and the known π* value for the gas phase, the LSER equation predicted ΔG in the gas phase to be -0.31 kcal mol-1, which agrees with -0.35 kcal mol-1 estimated from DFT-D calculations.