Solvation thermodynamics of simple crown ethers by the XRISM method

In this work, the solvation thermodynamics of the D_3d and C_i symmetry conformations of 18-crown-6 are investigated by way of extended reference interaction site model (XRISM) calculations. A broad range of solvent conditions are explored; viz., water, carbon tetrachloride, and acetonitrile at 25°C and 1 atm. We find that the relative free energies of solvation between the D_3d and C_i conformers display a strong dependence on solvent type with the D_3d structure becoming increasingly favored as solvent polarity increases. Our study of acetonitrile shows that electrostatic effects alone (with no hydrogen bonding) are sufficient to stabilize the D_3d conformer in solution. In addition to energetics, the structuring of the solvent in the vicinity of the solute is examined. We also consider the solvation of dibenzo-18-crown-6. Our results show that the attachment of lateral groups to the crown perturbs the solvent structure in largely a local manner.