Chemistry Reference
In-Depth Information
3 Instrumentation .............................................................................. 226
3.1 Microwave Sources . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 226
3.2 Microwave Waveguides . . . . . . . . ...................................................... 227
3.3 Probes Used for DNP Experiments ................................................... 228
4 Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
4.1 Applications to Small Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 228
4.2 Applications to Biomolecules . ....................................................... 234
4.3 Magnetic Resonance Imaging . . . . . . . . . ............................................... 235
4.4 Multidimensional Time-Domain Experiment for DNP . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
5 Future Perspectives .......................................................................... 239
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 239
1
Introduction
Nuclear Magnetic Resonance (NMR) is an important spectroscopic tool for the
identification and structural characterization of molecules in chemistry and bio-
chemistry. The most significant limitation of NMR spectroscopy compared to other
spectroscopic techniques is its relatively low sensitivity, which thus often requires
long measurement times or large amounts of sample, typically half a milliliter (mL)
of sample at rather high concentrations. The origin of low sensitivity in NMR is
well known to be due to the small magnetic moment of nuclear spins, which yields
small Boltzmann polarizations and weak absorption signals. One of the ways to
overcome this low signal-to-noise ratio is to enhance the signal by the creation of
hyperpolarized transitions. This can be achieved by a process which was named
dynamic nuclear polarization (DNP), where the polarization of high-gyromagnetic
ratio (
) electrons is transferred to the surrounding nuclei using microwave (MW)
irradiation.
In 1953, Albert Overhasuser [
1
] first proposed that it was possible to transfer
polarization to nuclei from electrons in metals by saturating the electron transition.
This idea was not widely accepted until experimentally verified by Carver and
Slichter with low field (3 mT) experiments performed on lithium metal and other
materials with mobile electrons [
2
,
3
]. This was soon expanded to be applied to
solid dielectrics by Abragam and Proctor [
4
]. Extension of electron-nuclear and
other high polarization transfer experiments involving noble gases, parahydrogen,
semiconductors, or photosynthetic reaction centers [
5
-
10
] to contemporary solid-
state and solution experiments is very appealing, since it could significantly
enhance the sensitivity in a variety of NMR experiments. In particular, the theoreti-
cal enhancement for electronuclear polarization transfers is approximately (
g
g
H
),
where the ratio is 660, making the gains in sensitivity ideally very large. Accord-
ingly, during the 1960s and 1970s, there were extensive efforts to perform electron
nuclear polarization transfer in liquids and solids. In 1980s, work has been carried
out to couple DNP to magic-angle-spinning solid-state NMR (MAS-ssNMR).
This concept of nuclear polarization enhancement, originally proposed by
Overhauser in 1953, was first experimentally demonstrated in metals and subse-
quently in liquids, which are two distinct types of systems with mobile electrons.
g
e
/
