Chemistry Reference
In-Depth Information
3.3 Probes Used for DNP Experiments
The probes for DNP experiment can be designed with or without a resonant structure.
For liquid state DNP experiments, the TE
102
rectangular EPR cavity is used for low
microwave frequencies polarization, while the cylindrical TE
011
cavity is used for
high frequencies [
45
,
46
]. In addition, an external RF coil with a slotted cavity is
required in a high frequency electron nuclear double resonance (ENDOR) setup to
avoid mounting the NMR RF coil inside the resonator. In 1965, Hyde proposed a
double resonance probe for ENDOR experiments, which was widely applied later
[
47
]. For MAS experiments, the microwave irradiation is applied vertically to the axis
of the rotor in the present design, instead of parallel in the past. Due to space
considerations, the microwaves are often placed between the turns of the NMR coil
in high frequency situations. The variable tuning circuits are positioned outside the
probe and wire transmission is used for the RF power irradiation. This design ensures
that the probe also works under low temperature conditions.
4 Applications
NMR is an important spectroscopic tool for the identification and structural char-
acterization of molecules in chemistry and biochemistry. The most significant
limitation of NMR compared to other spectroscopic techniques is its intrinsically
low sensitivity, which thus often requires long measurement times or large amounts
of sample. DNP has been shown to be an effective method to increase the nuclear
spin polarization in inorganic, organic, and biological materials; hence DNP has
become an attractive technique to boost the sensitivity of NMR signals allowing
NMR spectra of small amounts of samples from natural sources or from chemical
synthesis to be readily acquired. Perhaps even more interestingly, the availability of
the entire hyperpolarized NMR signal in one single scan allows the measurement of
transient processes in real time, if applied together with a stopped-flow technique.
In combination with the methods to dissolve rapidly the polarized solid sample, it is
possible to obtain a solution of molecules containing hyperpolarized nuclei. This
has enabled new applications in NMR spectroscopy as well as medical applications
in MRI.
4.1 Applications to Small Molecules
By using incipient wetness impregnation of the solid samples with a solution of
organic radical species such as TEMPO or TOTAPOL, surface enhanced NMR
spectroscopy by DNP can be carried out to allow fast characterization of
functionalized solid surfaces. Polarization is transferred from the radical protons
of the solvent to the rare NMR active nuclei at natural abundance on the surface.
Lesage et al
.
[
48
] have applied surface enhanced NMR spectroscopy by DNP to
