Chemistry Reference
In-Depth Information
where
o
1
E
are the field strengths of the applied RF fields. In solid-state
NMR experiments, the radio frequency fields are strong so their excitation profiles
can cover the entire NMR spectrum and they can efficiently spin-lock both spin
species. Thus, the Hartman-Hahn condition can be fulfilled.
For DNP experiments, however, there is large inhomogeneous broadening of
high field EPR spectra and the spectral breadth
o
1
N
and
D
can usually exceed several
hundred megahertz. In other words, the microwave field magnitude power is one
order of magnitude smaller in comparison with the conditions found in ssNMR. The
condition of matching electron-nuclear cross polarization (eNCP) cannot be
satisfied by modification of the microwave field strength and the RF strength.
Therefore, off resonance effects of EPR must be considered in this case.
From thermodynamic theory, the signal enhancement by CP transfer between
I (high-
g
) and S (low-
g
) spins is given by
1
1
g
I
g
S
x ¼
(9)
þ
N
S
=
N
I
The efficient CP transfer between two spins is required so that the concentrations
of I is much more than S. In this case, when
N
S
N
I
,
x
g
I
g
S
;
and the full ratio
can be transferred. NMR-CP experiments with the HHCP
schemes are typically performed by transferring polarization from abundant high
g
I
=
g
S
g
spins I to dilute low
spins S. The condition of efficient CP transfer can be easily
satisfied. However, in DNP experiments, the concentration of unpaired electrons
from high
g
polarizing agents is about four orders of magnitude lower than the
concentration of low
g
1
H nucleus. Therefore, the enhancement factor
1 and
only a small amount of electrons can be transferred to
1
H. The pumping of CP
polarization is applied to improve the enhancement because the relaxation time of
the electron is much shorter than that of the nucleus.
g
x <
2.2.1
Integrated Solid Effect
Integrated Solid Effect (ISE) was first introduced by Henstra et al. [
17
]. It can
overcome the low efficiency of SE when the homogeneous width is much larger
than the nuclear Larmor frequency (
Do
0
N
), in which the polarization effect could
be canceled by simultaneous saturation of the forbidden transitions at
o
0
E
o
0
N
.
The ISE can preserve the polarization in the case of
Do
0
N
by inverting a
forbidden EPR transition prior to saturation of an allowed transition. This effect
can be achieved by using a selective inversion pulse after the irradiation on
resonance at
o
0
E
o
0
N
, or applying CW microwave irradiation at a fix frequency
