Water-mediated conformer optimization in benzo-18-crown-6-ether/water system.

The conformation of benzo-18-crown-6-ether (B18C6) and its encapsulation of water molecules in a supersonic beam are investigated by laser induced fluorescence (LIF), UV-UV hole-burning, IR-UV double resonance (IR-UV DR), and resonance-enhanced multiphoton ionization (REMPI) spectroscopy with the aid of density functional theory (DFT) calculations at the B3LYP/6-31+G* level. At least four B18C6 conformers and nine B18C6-(H2O)n (n = 1-4) clusters are identified in the supersonic beam. IR-UV DR spectra in the CH stretching region suggest that the four B18C6 conformers have different conformations from each other. In contrast, most of the nine B18C6-(H2O)n clusters have a very similar B18C6 conformation. IR-UV DR spectra in the OH stretching region provide quite clear pictures of the hydration networks formed on B18C6. In all four B18C6-(H2O)1 isomers, the water molecule is H-bonded to the two O atoms adjacent to the benzene ring in "bidentate" and "bifurcated" manners. One of the four B18C6-(H2O)1 isomers exhibits a large population, and further hydration networks are preferentially grown on this specific isomer.