Chemistry Reference
In-Depth Information
Thus, DNP is not a new area of scientific endeavor, but rather one undergoing
a transition from low to high magnetic fields and frequencies. This chapter outlines
the theoretical descriptions of DNP mechanisms followed by recent developments
in DNP instrumentations and applications. DNP has proven to be one of the most
effective methods to increase the nuclear spin polarization in inorganic, organic,
and biological materials. In combination with 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 Magnetic Resonance Imaging (MRI).
2 Theories of Dynamic Nuclear Polarization
In electron-nuclear based DNP experiments, it is required that the electron para-
magnetic resonance (EPR) spectrum be irradiated with microwaves that drive the
exchange of polarization between the electrons and the nuclear spins. In the case
of liquids, these transitions are based on the Overhauser effect and, in solids, other
mechanisms - the solid effect (SE), thermal mixing (TM), or the cross effect (CE) -
dominate the polarization transfer process. Since DNP experiments require irradia-
tion of the EPR spectrum, they were confined to relatively low magnetic fields
because of the paucity of high frequency microwave sources. In particular, the
microwave sources used in both the liquid and solid-state experiments were klystrons
that operate at 640 GHz, constraining DNP-MAS experiments to 660 MHz
1
H
frequencies. Thus, for DNP to be applicable to the higher fields employed in
contemporary NMR experiments, new instrumental approaches to produce higher
frequency microwaves are necessary.
A theoretical analysis of electron and nuclear system requires the quantum
mechanical representation. In a DNP experiment, the general static Hamiltonian
is written as
H
¼
H
EN
¼ o
0
E
E
Z
o
0
N
N
Z
þ
H
E
þ
H
N
þ
H
i
EN
þ
H
d
EN
(1)
¼ o
0
E
E
Z
o
0
N
N
Z
þ
K
SE
ð
E
Z
N
Z
þ
E
Y
N
Y
þ
E
X
N
X
Þþ
K
PSE
E
X
N
Z
where
H
E
and
H
N
are the Hamiltonians for electron and nuclear respectively.
H
EN
is
the hyperfine coupling, which can be separated into the isotropic hyperfine interac-
tion
H
i
EN
and the anisotropic dipolar coupling
H
d
EN
between electrons and nucleus.
The
H
EN
can be further expressed in a form where the coefficients
K
SE
and
K
PSE
denote the secular and pseudosecular hyperfine interactions.
o
0
E
and
o
0
N
are the
electron and nuclear Larmor frequencies.
DNP experiments can be classified based on the polarizing mechanisms. We will
discuss the continuous-wave (CW) and time domain polarization mechanisms in
the following section.
