Chemistry Reference
In-Depth Information
2.1 Continuous-Wave DNP Mechanism
Continuous wave DNP polarization-transfer mechanisms can be generally classi-
fied into four fields, namely the Overhauser Effect (OE), the Solid Effect (SE), the
Cross Effect (CE), and Thermal Mixing (TM). They have all been successfully
applied to both solid and liquid samples. It is reported that many DNP applications
have been performed at low magnetic fields based on these different polarization
effects.
2.1.1 Overhauser Effect
The OE DNP mechanism is a relaxation process which relies on the mixing of an
electron and a nuclear spin. More specifically, in liquid cases, these relaxation
processes are based on time-dependent dipolar and scalar interactions between
electrons and nuclei. These interactions are governed by molecular rotational and
translational motion in the dipolar case and by chemical exchange and fast relaxa-
tion in the scalar case [
11
]. In solid cases, the mobile electrons are required for the
OE relaxation process. These electrons can be offered by the conduction band of
metal or one-dimensional organic conductors. In addition, the condition
o
0
E
t <
1
(
, rotational correlation time of paramagnetic species) is required for OE to be
established. The latter condition is difficult to satisfy with large value of
t
o
0
E
.In
addition,
o
0
E
is in proportion with the strength of magnetic field. Therefore, the
efficiency of the OE polarizing mechanism sharply decreases at high magnetic
fields. However, the OE polarization is extremely useful for liquids, for it is the only
practical mechanism for the direct application of liquids. We will give the outlines
of the main OE features in a later section. A more detailed description can be found
in the recent publication and review by Hofer et al
.
[
12
].
Figure
1
shows the energy level diagram of the transitions responsible for the
OE.
W
E
and
W
N
are the rates for the EPR and NMR transitions, respectively.
W
0
is
the nuclear relaxation rate in the absence of electrons, while
W
0
and
W
2
are the rates
for the zero and double quantum transitions, respectively. In OE polarization, the
allowed transitions of EPR are saturated. The observed NMR signals are enhanced
by changing the nuclear spin population, which is induced by the zero and double
Fig. 1 Energy level diagrams
and main transition rates
for the Overhauser effect
(OE) DNP mechanism
