Solvent Attenuation of London Dispersion in Polycyclic Aromatic Stacking.

Abstract

Solvent competition for London dispersion attenuates its energetic significance in molecular recognition processes. By varying both the stacked contact area and the solvent, here we experimentally deconvolute solvent attenuation using molecular balances. Experimental stacking energies (phenyl to pyrene) correlated strongly with calculations only when dispersion was considered. Such calculations favoured stacking by up to -27 kJ mol-1 in the gas phase, but it was weakly disfavoured in our solution-phase experiments (+0.5 to +4.6 kJ mol-1). Nonetheless, the propensity for stacking increased with contact area and in solvents with lower bulk polarisabilities that compete less for dispersion. Experimental stacking energies per unit change in solvent accessible area ranged from -0.02 kJ mol-1 Å-2 in CS2, to -0.05 kJ mol-1 Å-2 in CD2Cl2, but were dwarfed by the calculated gas-phase energy of -0.6 kJ mol-1 Å-2. The results underscore the challenge facing the exploitation of dispersion in solution. Solvent competition strongly but imperfectly cancels dispersion at molecular recognition interfaces, making the energetic benefits difficult to realise.