Magic angle spinning is a useful way to remove the line broadening effects of the CSA and dipolar terms in the Hamiltonian. However, MAS may not completely remove hetero-nuclear dipolar couplings and in these cases MAS is combined with a technique known as decoupling.40 Decoupling is commonly used in both solution- and solid-state NMR spectroscopy to remove any hetero-nuclear couplings that are present, including J- coupling. Multiple different decoupling schemes have been developed and each is tailored towards the specific requirements of a particular NMR experiment and spectrometer.
The most basic form of decoupling is known as continuous wave (CW) decoupling and consists of the application of continuous RF irradiation to a hetero- nucleus during acquisition (see figure 22).
Figure 22
However, this technique is limited because the RF irradiation the entire acquisition period
restriction corresponds to a limit in the frequency range over which the decoup effective. This is not a problem for
shift range, such as the
with a large chemical shift range, such as the
decoupling techniques have been developed to deal with the situation of decoupling nuclei with large chemical shift ranges.
The most common modification of CW decoupling is known as two pulse phase modulated (TPPM) decoupling.
alters the basic CW scheme by app applied for an entire rotor period and shown in figure 23.
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22 The basic continuous wave (CW) decoupling sequence.
However, this technique is limited because the RF irradiation must remain on acquisition period and this restricts the strength of the RF
restriction corresponds to a limit in the frequency range over which the decoup
effective. This is not a problem for decoupling nuclei that have a very small chemical shift range, such as the 1H nuclei, but is when attempting to decouple the effect of nuclei with a large chemical shift range, such as the 13C and 19F nuclei
decoupling techniques have been developed to deal with the situation of decoupling nuclei with large chemical shift ranges.
The most common modification of CW decoupling is known as two pulse phase modulated (TPPM) decoupling.41 TPPM is still a form of continuous decoupling but alters the basic CW scheme by applying a series of back to back RF pulses
applied for an entire rotor period and has the opposite phase of the previous pulse
The basic continuous wave (CW) decoupling sequence.
must remain on throughout of the RF field used. This restriction corresponds to a limit in the frequency range over which the decoupling is nuclei that have a very small chemical when attempting to decouple the effect of nuclei F nuclei. More specialized decoupling techniques have been developed to deal with the situation of decoupling
The most common modification of CW decoupling is known as two pulse phase TPPM is still a form of continuous decoupling but back to back RF pulses. Each pulse is opposite phase of the previous pulse, as
This phase alternating
which the CW decoupling occurs
are many modifications of TPPM decoupling,
phase shift where the two pulses are exactly 180 degrees out of phase with each other and is known as X inverse
greatly increase the efficiency of
sequence, where the RF pulses no longer remain on for the entire duration of the acquisition period, would allow for the use of much higher RF powers.
The use of multiple state NMR spectroscopy
homo-nuclear dipolar interaction decoupling schemes used
designed to decouple nuclei with a very large chemical shift anisotropy
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Figure 23 The TPPM decoupling sequence.
ternating scheme is designed to increase the effective frequency range which the CW decoupling occurs, and therefore increases decoupling performance are many modifications of TPPM decoupling,42-47 but the most common
phase shift where the two pulses are exactly 180 degrees out of phase with each other and X inverse-X (XiX) decoupling.48 TPPM and its numerous modifications
se the efficiency of decoupling; however, a multiple
, where the RF pulses no longer remain on for the entire duration of the would allow for the use of much higher RF powers.
The use of multiple-pulse decoupling has been around for a long
spectroscopy and was originally used in solid-state NMR to decouple the nuclear dipolar interaction.31-32 One of the more specialized
used in solid-state NMR is the XY-16 sequence designed to decouple nuclei with a very large chemical shift anisotropy
effective frequency range over decoupling performance. There but the most common one employs a phase shift where the two pulses are exactly 180 degrees out of phase with each other and TPPM and its numerous modifications a multiple-pulse decoupling , where the RF pulses no longer remain on for the entire duration of the
would allow for the use of much higher RF powers.
a long time in solution- state NMR to decouple the of the more specialized multiple-pulse 16 sequence (figure 24) and it is designed to decouple nuclei with a very large chemical shift anisotropy.49
This sequence consists of with each pulse having a phase
synchronized implies that each pulse is
decoupling scheme does not require continuous use of much stronger R
and allow for the decoupling of nuclei over a chemical shift range o More advanced multiple
been developed and allow for the selective removal the Hamiltonian. These sequences
to control which terms of the Hamiltonian will be decoupled
allow for the re-introduction of terms which have been removed by MAS
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Figure 24 The XY-16 decoupling sequence.
This sequence consists of a train of 16 individual rotor-synchronized with each pulse having a phase (ϕ) that follows the XY-16 phase cycle
synchronized implies that each pulse is applied at the end of a rotor period
decoupling scheme does not require continuous RF irradiation and therefore permits use of much stronger RF powers. These powers can be anywhere from 100 to
he decoupling of nuclei over a chemical shift range o multiple-pulse decoupling schemes based on symmetry been developed and allow for the selective removal or re-introduction
Hamiltonian. These sequences, known as C or R sequences, not only
to control which terms of the Hamiltonian will be decoupled during acquisition, but also introduction of terms which have been removed by MAS
synchronized 180 degree pulses 16 phase cycle.50 The term rotor- applied at the end of a rotor period (τr). This
and therefore permits the anywhere from 100 to 125 kHz he decoupling of nuclei over a chemical shift range of the same order.
symmetry principles have introduction of specific terms in not only allow the user during acquisition, but also introduction of terms which have been removed by MAS.51-58
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