2 FUNDAMENTALS OF SOLID-STATE NMR MEASUREMENT
2.3 HIGH POWER DECOUPLING
When
1
H and
13
C are directly coupled, the dipole interaction with
1
H is about 20 kHz. In
order to completely eliminate the dipole interaction with the
1
H,
1
H high-power decoup-
ling of about 60-80 kHz is usually required. Since there is a strong dipole interaction be-
tween the protons in an organic compound, you cannot use the technique of broad-band
decoupling that is used in the solution NMR. Therefore, it is necessary to decouple with
the high-power RF pulse. High-power decoupling peculiar to the solid-state NMR is
called “Dipolar Decoupling.”
Since RF pulses used for the high-power decoupling are very strong, decoupling is car-
ried out only during data acquisition.
CAUTION
The probe may become damaged if the data-acquisition time is too long.
2.3.1 CW (Continuous Wave) Decoupling
The CW decoupling is a technique that has been used for many years. In solution NMR,
various decoupling sequences other than CW decoupling have been proposed. However,
the decoupling sequence of solution NMR has not demonstrated an effect in solid
high-resolution NMR for the following two reasons.
•
Since there is a strong homonuclear dipole interaction between the
1
H’s in a solid sam-
ple, a powerful artifact signal appears as a cycling sideband.
•
In a pulse sequence having a long cycle, the decoupling effect is decreased by inter-
ference with MAS.
Moreover, CW decoupling has been a reliable decoupling technique because it is not in-
fluenced by imperfections in pulses.
2.3.2 TPPM (Two Pulse Phase Modulation) Decoupling
TPPM is a decoupling sequence proposed by A.E.Bennett et al.
Reference: A.E. Bennett et al., J.Chem.Phys., 103, 6951-6958 (1995).
Although it is a very simple decoupling sequence that repeats two 180
°
pulses in which
the phase only changed a small amount, the line-narrowing effect demonstrated in het-
eronuclear
1
H decoupling is more than in CW decoupling.
TPPM decoupling is not for canceling the wide range off-resonance effect by using
weaker power like the decoupling sequence seen in solution NMR. Rather the decoupling
effect becomes better with increasing decoupling power. Therefore, when TPPM decoup-
ling is carried out, it uses the maximum decoupling power that can be used with the in-
strument the same as in CW decoupling.
The effect of TPPM decoupling depends on the flip angle
ϕ
of the phase and the pulse
width
τ
p
. The TPPM decoupling with certain parameters cannot demonstrate an effect in
comparison with CW decoupling. Moreover, since the decoupling effect depends on the
pulse width, the effect decreases in a probe with poor uniformity of the RF magnetic field.
In TPPM decoupling, the decoupling effect will decrease if the power is weaker.
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