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V. ALVIN SHUBERT, Argonne National Laboratory, Chemical Sciences and Engineering Division, 9700 South Cass Avenue, Argonne, IL 60439; CHRISTIAN W. MÜLLER, WILLIAM H. JAMES III AND TIMOTHY S. ZWIER, Department of Chemistry, Purdue University, West Lafayette, IN 47907-2084.
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Crown ethers are well-studied examples of flexible macrocycles with a high binding selectivity for substrates, especially cations. We investigated the conformational preferences of the singly and doubly complexed water clusters of the crown ethers benzo-15-crown-5~(B15C) and its amino-derivative 4'-aminobenzo-15-crown-5~(ABC) cooled in a supersonic jet expansion. The fluorescence excitation, resonance enhanced two-photon ionization (R2PI), UV-UV holeburning (UVHB), fluorescence-dip infrared (FDIR), resonant ion-dip infrared (RIDIR) and novel IR-IR-UV holeburning1 spectra allowed for the identification of two \mathrmB15\mathrmC--(\mathrmH2\mathrmO)1 conformers and one \mathrmABC--(\mathrmH2\mathrmO)1 conformer. These conformers are characterized by an all-planar arrangement of the atoms directly bound to the benzene ring in which the crown ether macrocycle opens up to a symmetric structure and accomodates a doubly and triply H-bonded \mathrmH2\mathrmO molecule in two distinct ways, respectively. Two \mathrmB15\mathrmC--(\mathrmH2\mathrmO)2 conformers and one \mathrmABC--(\mathrmH2\mathrmO)2 conformer were identified. One of the \mathrmB15\mathrmC--(\mathrmH2\mathrmO)2 conformers contains a macrocycle configuration identical to that found in the monohydrated clusters with an H-bonding topology in which the \mathrmH2\mathrmO molecules occupy both available sites simultaneously. The second \mathrmB15\mathrmC--(\mathrmH2\mathrmO)2 conformer is assigned to an H-bond pattern in which the two \mathrmH2\mathrmO molecules are concatenated to form an H-bonded bridge involving only three of the four available O--H-bonds~(see figure).
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(1) V.~A. Shubert and T.~S.~Zwier, J. Phys. Chem.~A , \textbf2007, 111 , 13283.