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Solvent Attenuation of London Dispersion in Polycyclic Aromatic Stacking
Alex Elmi1, Krzysztof M Bąk1, Scott L Cockroft1
1EaStCHEM School of Chemistry, University of Edinburgh, Joseph Black Building, Edinburgh, EH9 3FJ, U.K.
Angewandte Chemie (International Ed. in English)
|July 23, 2024
Summary
Solvent competition significantly reduces the impact of London dispersion forces in molecular recognition. Experiments show stacking energies are weak in solution, unlike gas-phase calculations, highlighting challenges in exploiting dispersion.
Area of Science:
- Physical Chemistry
- Supramolecular Chemistry
- Chemical Physics
Background:
- London dispersion forces are crucial in molecular recognition but are influenced by solvent interactions.
- Quantifying the energetic contribution of dispersion in solution-phase molecular recognition is challenging due to solvent competition.
Purpose of the Study:
- To experimentally determine and deconvolute the energetic contribution of London dispersion in molecular recognition processes.
- To investigate the effect of solvent competition on stacking energies by varying contact area and solvent properties.
Main Methods:
- Utilized molecular balances to experimentally measure stacking energies between phenyl and pyrene systems.
- Varied stacked contact area and employed different solvents (CS2, CD2Cl2) to assess solvent attenuation.
- Correlated experimental data with computational calculations that included dispersion terms.
Main Results:
- Experimental stacking energies in solution were weakly disfavored (+0.5 to +4.6 kJ/mol), contrasting with gas-phase calculations (-27 kJ/mol).
- Stacking propensity increased with contact area and in solvents with lower bulk polarizability, indicating reduced dispersion competition.
- Experimental stacking energies per unit area were significantly lower than calculated gas-phase values, with values ranging from -0.02 to -0.05 kJ/mol/Ų in solution.
Conclusions:
- Solvent competition strongly attenuates, but does not completely cancel, the energetic benefits of dispersion in molecular recognition.
- Exploiting dispersion forces for molecular recognition in solution remains a significant challenge due to competitive solvent interactions.
- Understanding and quantifying solvent effects on dispersion is critical for designing effective molecular recognition systems.
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