Coherence using pulsed field gradient

Fig. 2. Pulse sequence for selective reverse INEPT using pulsed field gradients to select the coherence transfer echo. The 180° pulse pair in the middle of the 2r delay is not normally needed for t < 50 ms, and the second proton 180° pulse and first t2 delay maybe omitted if a linear phase gradient in the resultant spectrum can be tolerated. The second field gradient pulse has an area (7c/th) times that of the first.

Coherence selection using pulsed field gradients... 

Coherence Selection Using Pulsed Field Gradients... 

Because of the favorable cross-peak multiplet fine-structure, the HSQC experiment offers superior spectral resolution over the HMQC (heteronuclear multiple quantum coherence) experiment [13, 14], On the other hand, the HMQC experiment works with fewer pulses and is thus less prone to pulse imperfections. The real advantage of the HSQC experiment is for measurements of samples at natural isotopic abundance and without the use of pulsed field gradients, since the HSQC experiment lends itself to purging with a spin-lock pulse. Spin-lock purging in the HMQC experiment... 

We can consider the state — 2IZSZ as an intermediate state in coherence transfer, just as we did in the analysis of populations (SPT) above. Often in INEPT-based experiments, this intermediate state is used as a way of cleaning up other coherences that are not desired. A pulsed field gradient (PFG, Chapter 8) is a way of temporarily messing up the shims, and this will destroy any magnetization that is in the x-y plane. The intermediate state —2IZSZ is not affected, however, because there is no net magnetization in the x-y plane. After... 

We saw in Chapter 8 how a selective 180° pulse can be placed between two gradients of the same sign and duration to give a pulsed field gradient spin echo (PFGSE) that not only selects the desired coherence but also destroys any other coherences. First, we use a hard 90° pulse to create coherence on all spins, and then the first gradient twists the coherence into a helix (Fig. 8.21). The selective 180° pulse reverses the direction of twist in... 

These experiments use 1H decoupling during most parts of the pulse sequence, which increases the lifetime of coherences.31 The purging of the residual AP component of doublets is achieved by a combination of an adiabatic inversion and a low-level pulsed field gradient. 

COSY can be combined with a z filter (see Section 9.5) into z-COSY, which is occasionally used to produce absorption phase spectra. A 90°, T, 90° sequence replaces the second 90° pulse in the usual COSY sequence, just as in DQF-COSY (see Fig. 12.6a). In this case, however, pulse 2 rotates in-phase magnetization to the z axis, where it is stored for the short r period and restored to the xy plane by pulse 3. Suitable phase cycling or imposition of a pulsed field gradient eliminates the coherences remaining in the xy plane during r, so that the final... 

As usual, suitable phase cycling and/or use of pulsed field gradients is critical to avoid the detection of undesired coherences. 

BIRD-HMQC. The most difficult aspect of implementing the HMQC experiment is the suppression of signals from protons attached to C (the center-band or single quantum coherences) in favor of the protons attached to C (the satellites or double quantum coherences). The use of pulse field gradients (PFG, Section 6-6) is the most effective technique, but relatively few spectrometers are equipped with the hardware required for their generation. Fortunately, there is an effective alternative for the suppression of center bands by means of the BIRD Bilinear Rotation Decoupling) sequence, which is outlined by the vector... 

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