RAMAN-SPECTROSCOPIC STUDY OF THE CONFORMATIONAL EQUILIBRIA IN 18-CROWN-6 IN WATER - PREDOMINANT D(3D) CONFORMATION

The conformational equilibria in 18-crown-6 in water have been studied by Raman spectroscopy. The Raman bands for aqueous solutions of 18-crown-6 with 20-100 wt% concentrations have been assigned to particular conformational forms or conformational fragments of the crown ring. The spectral changes with varying concentration indicate that the conformational equilibria in 18-crown-6 depend significantly on the composition of the solution. In dilute aqueous solutions, 18-crown-6 molecules predominantly adopt the D3d conformation. Conformations other than D3d also coexist at low concentrations, although their populations are small. These experimental results are in good agreement with the theoretical prediction by Monte Carlo and molecular dynamics simulations. In the neat liquid and concentrated aqueous solutions, 18-crown-6 molecules adopt diverse conformations.