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UNTERRICHT
tautomeric forms (keto/enol) of 1-n-butyl-5-(4-nitrophenyl) barbituric
acid are found, self-aggregation due to complementary
hydrogen-bonding favours only one tautomer in the solid-state.
The resulting dimeric complex tolerates treatment with rather
strong bases (e.g. 1,8-diamino-naphthalene) but is broken in
the presence of adenine-mimicking diacetamido-pyridin. After
tautomeric rearrangement of 1-n-butyl-5-(4-nitrophenyl) barbituric
acid, a 1:1 complex with diacetamido-pyridin is formed.
Using 60 kHz MAS and our highest available magnetic fi eld
( 1 H, 850.1 MHz@20 T), we could unambiguously identify the
structure of this complex. Previous attempts at lower fi elds and
lesser MAS rotation frequency (e.g. 30 kHz MAS, 1 H, 700.1
MHz@16.4 T) did not provide suffi cient resolution, thus demonstrating
the benefi ts from ongoing developments of NMR
hardware. 55 The interpretation of experimental chemical shift
data was supported by ab initio DFT-based quantum chemical
computations. 56
Figure 13: 1 H- 1 H double-quantum MAS NMR spectrum of the chromophordiacetamido-pyridin-complex
at 850.1 MHz and 60 kHz MAS. The relevant
OH/NH-crosspeaks are highlighted. At the right, a DFT-optimized model geometry
of the complex is given. (Gaussian03//BLYP/6-311++G(2df,2pd)). 56
Though for many host-guest complexes 57 structural data from
single-crystal X-ray analysis is available, proton positions are
commonly refi ned according to known geometries rendering
them rather ill-defi ned. In this case, ab initio DFT-based structure
refi nement is mandatory to account for experimentally observed
1 H chemical shifts. 58 Furthermore, we could show that
a comparison of X-ray structures with NMR data is only feasible
at comparable temperatures, particularly in cases where
dynamic effects such as exchange of protons along hydrogen
bonds occur. This, for instance, has been observed for α-truxillic
acid where the 1 H MAS NMR spectrum displayed only an
averaged peak at 14 ppm for the carboxylic acid proton, while
the respective 13 C-CPMAS spectrum revealed two carbonyl car-
Figure 14: (left) Static 2 H NMR spectrum of α-truxillic acid. The quadrupolar
splitting of 117 kHz indicates a two-site jump in the fast limit. (right) Xray
structure at 350 K revealing 50:50 occupancies of the proton along the
OH---O=C hydrogen bond. 59
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BUNSEN-MAGAZIN · 11. JAHRGANG · 2/2009
bons resonating at 178.9 ppm and 182.3 ppm, respectively.
Since 2 H nuclear quadrupolar couplings are a sensitive probe
for the asymmetry of hydrogen bonds, we acquired a static
2 H NMR spectrum yielding a (quadrupolar) Pake pattern with
a quadrupolar splitting of 117 kHz (cf. fi gure 14), which indicates
a two-site jump of the proton along the hydrogen bond
in the fast limit. This could be confi rmed by high-temperature
X-ray analysis at 350 K revealing 50 : 50 occupancies of the
corresponding COOH proton within the OH---O=C hydrogen
bond. 59
The structure and dynamics of the protons is particularly interesting
in proton conductors. In a recent study, we applied
solid state 1 H (and 2 H) -NMR to a promising class of almost
anhydrous proton conductors that are aimed at generating
improved membranes for high-temperature fuel cell operation.
In anhydrous systems, proton conduction should not rely
on the presence of water or other small molecules (like in
the industry benchmark compound Nafi on Ò ), but should originate
from so-called structural diffusion among the protogenic
groups. 53 One representative compound is given by polyvinyl
phosphonic acid (PVPA). The PO(OH) 2 protons resonate at
10.6 ppm, thus indicating hydrogen-bonding of moderate
strengths. 1 H MAS NMR spectra at increasing temperatures
display substantial line-narrowing for this peak while its chemical
shift remains nearly constant indicating a fast exchange
among non-equivalent proton sites. In addition, static 2 H NMR
spectra at variable temperatures ranging from 250 to 390 K
were also recorded, and revealed a complete averaging of the
initial (quadrupolar) Pake pattern to an isotropic line. In both
cases, apparent activation energies were extracted assuming
an Arrhenius-type behaviour of the proton mobility. From 1 H
NMR data, an apparent activation energy of 25 kJ/mol is obtained
while 2 H NMR data yields 38 kJ/mol, which is in good
agreement with an apparent activation energy of 45 kJ/mol
extracted from an analysis of temperature-dependent proton
conductivities in the bulk sample. 60 Nevertheless, the almost
complete averaging of the quadrupolar Pake pattern at high
temperatures is not easy to explain. A geometric scenario
which could lead to such an averaging would be that the principal
axis of the quadrupolar tensor of the jumping deuteron
is aligned at the magic angle with respect to the magnetic
fi eld, a condition that is obviously diffi cult to achieve in a bulk
polymer sample. A Car-Parrinello Molecular Dynamics (CPMD)
simulation on a model fragment of PVPA, however, not only
accounts for the average chemical shift, 61 it also revealed a
broad distribution of quadrupolar tensor orientations for the
deuterons after a run of only 15 ps or so. The average quadrupolar
splitting resulting from this is only 10% of the coupling in
the rigid limit, thus indicating a complex exchange behaviour
of the protons. 61
3.2. SHAPE CONTROL
Molecular recognition due to hydrogen-bonds also governs
the selective incorporation of small solvent molecules into
calix[4]hydrochinone-based nanostructures. The extended tubular
assemblies are formed from a monomer-water mixture
where the water molecules act as bridging units. 62