Chemistry Reference
In-Depth Information
Fig. 3 Illustration of the electron (a) and nuclear (b) spin effective fields. The effective fields
belonging to the EPR and NMR transitions are no longer equal.
N
are the resonant offsets
of electrons and nuclei, while
A
e
and
A
n
are the hyperfine coupling constant of electrons and nuclei.
o
O
E
and
O
B;
eff
1
N
) are the unequal effective microwave fields on different resonant
offsets of electrons (nuclei).
y
EA=EB=NA=NB
are the angles between the effective microwave fields on
electrons/nuclei and the
z
dimension
A;
eff
1
E
B;
eff
1
E
(
o
A;
eff
1
N
and
o
and
o
q
ðd
E
þ
q
ðd
E
2
2
2
1
E
2
1
E
A
e
=
2
Þ
þ o
þ
A
e
=
2
Þ
þ o
q
ðd
N
þ
q
ðd
N
2
2
2
1
N
2
1
N
¼
A
n
=
2
Þ
þ o
þ
A
n
=
2
Þ
þ o
(10)
where
d
E
and
d
N
are the resonant offsets of electron and nuclear spectra, respec-
tively.
o
1
N
represent the RF field strengths and
A
e
and
A
n
the hyperfine
coupling constants. For NMR-CP experiments, the matching condition is given by
the equation
o
1
E
and
q
o
q
o
2
2
1
E
1
n
1
E
d
N
þð
A
e
=
2
Þ
o
þð
A
e
=
2
Þ
(11)
in the case that the microwave irradiation is on resonance and the hyperfine
coupling is arbitrary. The expected polarizing region is determined by hyperfine
couplings.
2.3 Polarizing Agents
Polarizing agents are widely used in DNP experiments. The choices of these agents
are strongly dependent on a number of factors, such as the width of the EPR
spectrum, the radical solubility and toxicity, the radical reactivity, the temperature
dependence of relaxation times, etc. Either exogenous or endogenous radicals
can be used as polarizing agents. Figure
4
shows five polarizing agents which
