The structure of host-guest complexes between dibenzo-18-crown-6 and water, ammonia, methanol, and acetylene : Evidence of molecular recognition on the complexation
Complexes of dibenzo-18-crown-6 (DB18C6, host) with water, ammonia, methanol, and acetylene (guest), formed in supersonic jets, have been characterized by using various spectroscopic methods: laser induced fluorescence (LIF), UV-UV hole-buning (UV-UV HB), and IR-UV double resonance (IR-UV DR) spectroscopy. Firstly, we reinvestigated the conformations of bare DB18C6 (species m1 and m2) and the structure of DB18C6-H2O (species a) [R. Kusaka, Y. Inokuchi, T. Ebata, Phys. Chem. Chem. Phys., 2008, 10, 6238] by measuring the IR-UV DR spectra in the region of the methylene CH stretching vibrations. From the IR-UV DR spectra, it was found that the IR spectral feature of the methylene CH stretch of DB18C6-H2O is clearly different from those of bare DB18C6 conformers, suggesting that DB18C6 changes its conformation when it forms the complex with a water molecule. With the aid of Monte Carlo simulation for extensive conformational search and density functional calculations (at B3LYP and M05-2X/6-31+G* levels), we reassigned the species m1 and m2 to the conformers belonging to C1 and C2 symmetry, respectively. On the other hand, we confirmed the DB18C6 part in the species a to be “boat” conformation, which is an unstable structure as the bare form. Secondly, we identified nine, one, and two species for DB18C6-ammonia, -methanol, and -acetylene complexes, respectively, by the combination of LIF and UV-UV HB spectroscopy. From the IR-UV DR spectra in the methylene CH stretching region, similar conformational change was identified in the DB18C6-ammonia complex, but not in the complexes with methanol or acetylene. The structures of all the complexes were determined by analyzing the electronic transition energies, exciton splitting, and IR-UV DR spectra in the region of the OH, NH, and CH stretching vibrations. In the DB18C6-ammonia complexes, a NH3 molecule is incorporated into the cavity of the boat conformation by forming bifurcated and bidentate hydrogen-bond (H-bond), similar to the case of the DB18C6-H2O complex. On the other hand, in the DB18C6-methanol and -acetylene complexes, methanol and acetylene molecules are simply attached to the C1 and C2 conformers, respectively, with their original conformations retained. From the difference of the DB18C6 conformation depending on the type of the guest molecules, it is concluded that DB18C6 distinguishes water and ammonia from methanol and acetylene when it forms complexes, depending on whether guest molecules have an ability to form bidentate H-bonding.
Physical Chemistry Chemical Physics
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Royal Society of Chemistry
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Graduate School of Science