Chemistry Reference
In-Depth Information
inhomogeneously and homogeneously broadening at low magnetic field [
18
]. Since
both CE and TM have recently been used at high magnetic fields, the theoretical
framework describing these effects needs to be fully established [
19
-
23
].
Griffin and co-workers [
22
] proposed that the CE is defined as a three-spin
process, involving the interaction between two dipolar coupled electrons with EPR
frequencies
o
E
01
and
o
E
02
that satisfy the relation
o
0
E
2
o
E
01
¼ o
0
N
(7)
When the EPR lines are broadened by the inhomogeneous anisotropy, this is
the dominant mechanism. The electrons are weakly coupled via electronic cross
relaxation. Theoretically, the CE requires that the inhomogeneous breadth
O
is larger
than the nuclear Larmor frequency
o
0
N
to make two effective EPR resonance fre-
quencies. Meanwhile, the homogeneous width must satisfy
D < o
0
N
. The biradical
chemical polarizing agents can improve the polarization effect of DNP systems to
achieve this condition by a dipolar coupling between its two electrons. In contrast, the
TM polarization mechanism contains homogeneously broadened EPR lines, where
D > o
0
N
is satisfied. This condition requires a high concentration of polarizing agent
at highmagnetic fields, which will restrict the resolution in anMAS NMR experiment.
Figure
2
shows the thermal equilibrium spin population for a three-spin system.
Generally there is no degeneracy present. When an appropriate polarizing agent is
used, the energy levels |IV
become degenerated
(Fig.
2a
). Irradiation of EPR transition and CE transitions leads to positive (Fig.
2b
)
or negative enhancement (Fig.
2c
) of the nuclear polarization.
The signal enhancements of CE/TM are scaled with the magnetic field strength
because the line width of the EPR spectra decreases along with the increasing
strength of the magnetic field. Despite this drawback, the CE/TM polarizing
mechanisms have been widely applied to polarize biological solids at high magnetic
fields [
24
,
25
].
The TM effect is less efficient compared to the CE. The TM can be described by
a series of interacting systems: the electron Zeeman system (EZS), the electron
dipolar system (EDS), and the nuclear Zeeman system (NZS) [
19
].
>
and |V
>
or |VI
>
and |III
>
2.2 Pulsed DNP
S.R. Hartmann and E.L. Hahn reported that pulsed DNP experiments based on
coherent polarization transfer such as the Hartmann-Hahn cross polarization
(HHCP) are more efficient and do not show a field dependence of the polarization
transfer with increasing magnetic field strengths.
HHCP is a fundamental technique widely used in ssNMR spectroscopy. With
this mechanism, the polarization of one nucleus can be transferred to another
nucleus [
26
,
27
]. This transfer requires the rotating frame Hartmann-Hahn
matching condition to be satisfied:
o
1
N
¼ o
1
E
(8)
