Water-Induced Structural Changes in Crown Ethers from Broadband Rotational Spectroscopy.

The complexes of 12-crown-4 ether (12C4) with water, generated in a supersonic jet, have been studied using broadband Fourier transform microwave spectroscopy. Three 1:1 and one 1:2 clusters have been observed and their structures unambiguously identified through the observation of isotopologue spectra. The structures of the clusters are based on networks of O-H···O and C-H···O hydrogen bonds. The most abundant 1:1 cluster is formed from the most stable S4 symmetry conformer of 12C4, even though it is not the energetically favored water complex. Interestingly, the structures of the most stable water cluster and the other remaining observed 1:1 and 1:2 complexes are formed from the second or the fifth most abundant conformers of 12C4. This shows the existence of a mechanism that changes the conformation of 12C4 so that the host-guest interactions can be maximized, even for a "soft" ligand like water.