Knowledge Bank

\begin{wrapfigure}{r}[0cm]{3.5cm} \vspace{-2.0cm} \hspace*{-1.7cm} \epsfig{width=0.4\textwidth,file=B15C-W2-test.eps} \end{wrapfigure} 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 holeburning$\mathrm{<mrow\; data-mjx-texclass="ORD">1</mrow>}$ spectra allowed for the identification of two $B15C$--$(H2O)1$ conformers and one $\mathrm{ABC}$--$(H2O)1$ 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 $H2O$ molecule in two distinct ways, respectively. Two $B15C$--$(H2O)2$ conformers and one $\mathrm{ABC}$--$(H2O)2$ conformer were identified. One of the $B15C$--$(H2O)2$ conformers contains a macrocycle configuration identical to that found in the monohydrated clusters with an H-bonding topology in which the $H2O$ molecules occupy both available sites simultaneously. The second $B15C$--$(H2O)2$ conformer is assigned to an H-bond pattern in which the two $H2O$ molecules are concatenated to form an H-bonded bridge involving only three of the four available O--H-bonds~(see figure). \vspace*{0.2cm} (1) V.~A. Shubert and T.~S.~Zwier, \textit{J. Phys. Chem.~A}, \textbf{2007}, \textit{111}, 13283.