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Figure 15: Molecular recognition of solvent molecules in calix[4]hydrochinone-based nanotubular assemblies. 64
Since only one of the two hydrogen-bond acceptor sites is used
within the tubes, the free coordination site is available for molecular
recognition of solvents. We could demonstrate that (mono-)
alcohols are preferably bound even in the presence of acetone
which is a suitable solvent for the nanotubes. The presence of
the respective solvent molecule within the assembly is clearly
revealed by characteristic shifts of the corresponding solvent 1 H
NMR signal. Notably, compared to the free solvent, incorporated
molecules experienced substantial downfi eld shifts (even to
negative ppm values) which could be rationalized with the help
of a so-called Nuclear Independent Chemical Shift (NICS) 63 map
obtained from an ab initio DFT computation. 23 Though acetone
facilitates formation of such nanotubes, it can be completely
replaced by 2-methyl-2-propanol or 2-propanol, while cyclohexanone
is rather poorly and pyridine not at all incorporated. This
fi nding particularly emphasizes that molecular recognition does
not solely rely on the mere presence of suitable functional
groups but also on additional factors like van-der-Waals forces
or mobility of the respective guest molecule. 64
Selectivity on a molecular level also constitutes the basis of
specifi c functions of natural receptors. Due to the size and
complexity of the objects of interest, much insight can be
gained on rather simple model compounds. Therefore, based
on tweezer complexes 65 that form host-guest structures with
electron-defi cient aromatic guests (“molecular pincettes”), we
UNTERRICHT
studied the infl uence of hydrogen-bonding and p-p-interaction
to neighbouring molecules on the respective 1 H chemical shifts
of the guest molecules. Unambiguous peak assignment and interpretation
of the 1 H MAS spectra of such complexes is based
on both 1 H- 1 H DQ MAS and 1 H- 13 C chemical shift correlation
spectra as well as ab initio chemical shift computations at high
level of theory employing a so-called linear scaling method. 66 In
order to accurately predict the respective 1 H chemical shifts of
the complexes, packing effects had to be taken into account,
thus refl ecting perturbing local fi elds present at the site of the
guest molecule. Indeed, the power of a combined experimental
and computational approach is increasingly applied in the fi eld
of “NMR crystallography” 67 where inter- and intramolecular
1 H- 1 H distances or 1 H chemical shifts (derived from NMR) are
used as input for computational methods.
3.3. COLUMNAR STRUCTURES
In addition to hydrogen-bonding, the 1 H chemical shifts are
very sensitive to ring currents associated with p-electrons in
aromatic moieties. 68 While in solution, the isotropic molecular
tumbling removes most of the p-shifts, the protons are exposed
to the p-electrons of nearby either intra- or intermolecular aromatic
moieties in the solid. The latter information is particularly
valuable for elucidating supramolecular structures. 69 Indeed,
Figure 16: Molecular tweezer complex with tetracyanobenzene guest: 1 H- 1 H double-quantum MAS NMR spectrum at 700 MHz and 30 kHz MAS. Selected
slices reveal the correlation peaks among the resonances at (a) 4.1 and 2.0 ppm, (b) 7.0 and 2.0 ppm and (c) 7.0 and 4.1 ppm, respectively. 66
69