doi:10.1016/j.cplett.2004.02.098
Copyright © 2004 Elsevier B.V. All rights reserved.
Solvent molecules trapped in supramolecular organic nanotubes: a combined solid-state NMR and DFT study
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Anke Hoffmann a, Daniel Sebastiani a, Erli Sugiono a, Sungoo Yun b, Kwang S. Kim b, Hans Wolfgang Spiess a and Ingo Schnell 
, 
, a
a Max-Planck-Institute for Polymer Research, NMR Division, Postfach 3148, 55021, Mainz, Germany
b Division of Molecular and Life Sciences, Department of Chemistry, National Creative Research Initiative Center for Superfunctional Materials, Pohang University of Science and Technology, Pohang 790-784, South Korea
Received 24 November 2003;
Revised 16 February 2004.
Available online 20 March 2004.
Abstract
When crystallised from water/acetone mixtures, calix[4]hydroquinone (CHQ) forms supramolecular nanotubes which we studied by NMR spectroscopy and DFT calculations, using the 1H and – for the first time – 2H chemical-shift resolution achievable in the solid state under fast (30 kHz) magic-angle spinning. Acetone molecules are trapped in the bowl-shaped CHQ molecules inside the tubes and hydrogen-bonded to an extended chain of hydrogen bonds, which is formed by CHQ and water molecules along the tube axis. Both water and acetone molecules occupy well-defined average positions, but undergo fast reorientation motions during which their protons interchange their positions.
Fig. 1. (a) Bowl-shaped conformation of calix[4]hydroquinone (CHQ), (b) stabilised by a ring of four hydrogen bonds at the bottom of each bowl. (c,d) Self-assembly of the CHQ bowls into nanotubes. (e) Crystal structure of the CHQ nanotubes which have a 17 Å × 17 Å cross-section with a 6 Å × 6 Å pore (with the van der Waals volume excluded).
Fig. 2. 1H solid-state NMR spectra of CHQ nanotubes. (a) Spectrum recorded under 30 kHz MAS. (b) Calculated 1H spectrum of the CHQ nanotubes without further guest molecules. The two protons of the water molecules in the 1D hydrogen-bonded chain give rise to two separated lines which have been averaged in order to account for fast motional processes in which the two protons interchange their positions. A broadening has been applied to all resonances by means of appropriate Gaussian convolutions. (c) 1H–1H double-quantum filtered (DQF) spectrum, recorded under 30 kHz MAS using one rotor period for excitation and reconversion of 1H–1H DQ coherences.
Fig. 3. The 30-kHz MAS spectra of CHQ nanotube crystals grown from solvent mixtures with one component deuterated. (a) 1H MAS spectrum of CHQ tubes grown from D2O/acetone. (b) 1H MAS spectrum and (c) 2H MAS spectrum of CHQ tubes grown from H2O/D6-acetone.
Fig. 4. 1H–1H double-quantum (DQ) NMR spectrum of CHQ nanotube crystals, recorded under 30 kHz MAS using one rotor period of back-to-back recoupling for excitation and reconversion of the DQ coherences.
Fig. 5. (a) Representation of a bowl-shaped CHQ molecule taken out of the nanotube crystal structure (i.e., with hydrogen-bonded water molecules attached). An acetone molecule is trapped inside the CHQ bowl with its carbonyl oxygen hydrogen-bonded to an available water proton. (b) Representation of a unit cell of the CHQ nanotube filled with acetone molecules. (c) Experimental 1H spectrum of CHQ nanotube crystals. (d) Calculated spectrum using the CHQ nanotube crystal structure filled with acetone molecules (shown in b). All chemical shifts of methyl and water protons have been averaged. Different Gaussian line broadening was applied for the signals of rigid (nanotube) and mobile (water, acetone) atoms.
Table 1. Experimental and calculated values of the 1H NMR chemical shifts of the CHQ tubes, along with those of the trapped acetone
The presence of several values for a single proton species indicates topologically different protons of the same molecule.
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