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. 2016 Feb 1;7(2):1401-1407.
doi: 10.1039/c5sc03550c. Epub 2015 Nov 17.

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

Bright U Emenike  1 , Sara N Bey  1 , Brianna C Bigelow  1 , Srinivas V S Chakravartula  2
Affiliations

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

Bright U Emenike et al. Chem Sci. .

Abstract

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 - 0.68β - 0.1π* + 0.09δ]-expresses ΔG 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 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.

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Figures

Fig. 1
Fig. 1. Scheme showing folded and unfolded conformational states of molecular torsional balance 1 and 2. For structural details, see ref. 16 for related single crystal structures.
Scheme 1
Scheme 1. Synthetic route to molecular torsion balances 1 and 2.
Fig. 2
Fig. 2. Structures of molecular balances 1–4 used in the solution studies of CH–aryl interactions.
Fig. 3
Fig. 3. (a) Correlation plot of experimental ΔGexp with solvent ET(30) polarity scale. (b) Linear solvation energy relationship constructed with Kamlet–Taft solvent parameters.
Fig. 4
Fig. 4. The maxima (Emax) and minima (Emin) in the AM1 molecular electrostatic potential surfaces of 24 solvent molecules.
Fig. 5
Fig. 5. Molecular electrostatic potential surface plotted on the van der Waals' surface of cyclohexane calculated using AM1 level of theory.
See this image and copyright information in PMC

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