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2.5
CROSS POLARIZATION
The Cross-Polarization (CP) method is the technique most generally used in solid-state
NMR. The CP method is usually called the "CPMAS" method because it is used in com-
bination with MAS.
Usually, CPMAS measurement has the following advantage when comparing it with the
single-pulse measurement.
• Improvement of the SN ratio
• Shortening of repetition time (improvement in accumulation efficiency)
2.5.1
Principle of Cross Polarization
The
H nucleus and X nucleus are irradiated by RF simultaneously, and the spin lock is
1
applied. The spin lock is a technique that irradiates a specific nucleus with a high-power
radio frequency at the resonance frequency to arrange the magnetization of the nucleus in
the direction of the B
spin-lock frequency of these two nuclei become equal, exchange between the magnetiza-
tions happens. This is called the "Hartmann-Hahn conditions." If we denote the B
netic field strengths for I and S nucleus as B
and γ
, respectively, the Hartmann-Hahn conditions can be expressed with the following
S
formula.
| γ
I
B
Be aware of the direction of the spin lock and magnetization when the magnetization of
the I and S nuclei reach equilibrium. When the signs of the γ ratio are the same, both
magnetizations are turned in the same direction. When the signs of the γ ratio are opposite,
the magnetizations are turned in opposite directions. For example, when observing the
magnetization transfer from
ratio is negative) are observed in reverse phase.
This method is often used for the magnetization transfer from
method improves the sensitivity of
urement can be repeated with approximately the relaxation time T
efficiency improves markedly.
The improvement of accumulation efficiency is
This method is applied for the rare spin system (
containing
NMECAXS_V50-SLD-2
2 FUNDAMENTALS OF SOLID-STATE NMR MEASUREMENT
magnetic field. When the B
1
I
| = | γ
S
B
S
|
1
1
1
H, the magnetizations of
Z
I
M
B
0
Fig. 2.2 Cross polarization of I and S nuclei
H.
1
magnetic field strengths of the
1
I
and B
S
and γ ratio of I and S nucleus as γ
1
1
13
C ( γ ratio is positive) and
Z
I
I
S
S
| γ
B
| ＝ | γ
B
|
1
1
S
M
I
B
1
13
C by γ
1H
/ γ
13C
= 4 times. Furthermore, since a meas-
13
C
1
(
T
T
1
1
15
N,
13
C,
1
or
S
B
1
1
H to
13
C. Using this
of
H, accumulation
1
1
H
.
)
29
Si and
31
P) of the spins 1/2
mag-
I
29
Si ( γ
2-5
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