BUNSENMAGAZIN - Deutsche Bunsengesellschaft für ...

DEUTSCHE BUNSEN-GESELLSCHAFT
troscopy. The basic pulse sequence for a 1 H- 13 C Heteronuclear
Multiple-Quantum Correlation (HMQC) experiment is depicted
in Figure 11. 48 As in homonuclear DQ-NMR, the fi rst half of the
REDOR recoupling is identifi ed as the excitation period, and
the second as the reconversion period. In between, HMQC evolution
occurs during t 1 . Evolution due to the 13 C chemical shift
that would lead to additional signal losses is fully refocused
by p-pulses during excitation, evolution and reconversion (in
simple words: its effect is undone).
Figure 11: Basic pulse sequence of REPT-HMQC. The I-channel is usually 1 H,
while the S-channel often refers to 13 C, 15 N or 31 P. REPT means recoupled
polarisation transfer and indicates that the pulse sequence works on initial
I-channel magnetisation, and thus does not require cross-polarisation. Setting
n = 0 corresponds to a recoupling time of 2t R . 48
Indeed, the absence of coherent line-narrowing pulse schemes,
which are often rather sensitive to experimental imperfections,
renders this experiment fairly robust. The recoupled polarisation
transfer (REPT) technique is highly versatile in that it
either allows recording of rotor-synchronized 1 H- 13 C chemical
shift correlation spectra or the determination of even weak 1 H-
13 C dipolar coupling constants by means of spinning sideband
analysis in the indirect dimension of the experiment. Again,
these sidebands are generated by rotor encoding of the reconversion
Hamiltonian. 46
3. SELECTED APPLICATIONS
The versatility of solid-state NMR spectroscopy to elucidate
both the structure and dynamics of (self-)organized possibly
functional aggregates is demonstrated based on selected examples
mainly taken from our laboratory. The rather recent
applications are sorted with respect to the major interaction
or driving force that governs the assembly into macro- or supramolecular
systems.
3.1. HYDROGEN-BONDING
Currently, much interest is focussed on the controlled, rational
design of well-ordered structures that are based on fairly weak
non-covalent interactions such as hydrogen bonds. Though
this approach in principle includes crystals (e.g. in the context
of crystal engineering), many hydrogen-bonded aggregates
crystallize rather poorly rendering single-crystal X-ray structure
analysis diffi cult. In contrast, ultrafast magic-angle-spinning
solid-state NMR is a useful and sensitive tool for the study
of such materials. In particular, the proton chemical shift allows
for a direct detection of local structural environments
UNTERRICHT
and facilitates an estimation of hydrogen-bonding strengths. 50
Indeed, protons involved in hydrogen-bonded structures typically
exhibit well-resolved 1 H chemical shifts, mainly between 8
and 20 ppm. Moreover, correlations between the 1 H isotropic
chemical shifts and the hydrogen-bond strength specifi ed by
O•••H or O•••O distances from single-crystal analyses have
been established. 51 Further resolution may be obtained from
1 H double-quantum 1 H NMR methods (see above) that not
only provide precise information on proton-proton distances on
length scales of up to 0.35 nm but also identify proton positions
in arrays of multiple hydrogen bonds. In this way, dynamic
processes involving molecular recognition 52 or formation of hydrogen-bonded
complexes 50 as well as proton conducting materials
53 can be investigated in detail.
In a rather early case-study, both the thermodynamics and
kinetics of the tautomeric rearrangements of ureidopyrimidinone-based
supramolecular polymers with quadruple hydrogen
bonds was elucidated. The thermally induced and irreversible
transition from pyrimidinone to pyrimidinol could be monitored
via temperature-dependent 2D- 1 H double-quantum spectra
since the two tautomeric structures are involved in different
arrays of quadruple hydrogen-bonds that can be distinguished
by their characteristic DQ peak pattern, thus facilitating an extraction
of the keto/enol ratio present in the sample. It was
found that the rearrangement is associated with an Arrhenius
activation energy of 145 � 15 kJ mol -1 , which contains contributions
from the breaking of hydrogen-bonds and subsequent
reorientation of the polymer chain. 54
Figure 12: Schematic representation of the thermally induced, irreversible
transition from pyrimidin-one to pyrimidin-ol and their characteristic distinguishable
DQ peak pattern. 54
Artifi cial chromophoric receptors for biologically active molecules
have attracted considerable attention from the viewpoint
of molecular recognition. Indeed, barbiturates show very selective
affi nities to binding adenine or its derivatives rendering
them of interest for possible diagnostic applications. By means
of 1 H- 1 H double-quantum and 1 H- 13 C chemical shift correlation
NMR, we obtained detailed insights into packing and (self-)aggregation
of the enolizable chromophor 1-n-butyl-5-(4-nitrophenyl)
barbituric acid that offers adjustable hydrogen-bonding
sites. While in solution, depending on the solvent, up to four
67