The Basic Pulse Sequence

The basis for diffusion measurements is the fact that magnetic field gradients can be used to indirectly label the position of spins through their Larmor frequency 

8) the first term is the phase shift due to the static magnetic field and the second term is the phase shift due to the magnetic gradient pulse applied along the z-direction. The next step is applying a 180° rf pulse, which inverts the sign of the precession and the phase as depicted in Fig. 6.1. At time N -F A a second identical pulse gradient is applied which cancels out the induced phase shifts, 

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The basic pulse sequence for the production of a spin-echo is illustrated in Fig. 2.1. The behavior of C vectors in a heteronuclear CH sys-... 

The basic pulse sequence employed in the heteronuclear 2D shift-correlation (or HETCOR) experiment is shown in Fig. 5.40. The first 90° H pulse bends the H magnetization to the y -axis. During the subsequent evolution period this magnetization processes in the x y -plane. It may be considered to be made up of two vectors corresponding to the lower (a) and higher (/3) spin states of carbon to which H is coupled. These two... 

In the previous sections, only the basic, non-gradient ID TOCSY pulse sequence, its experimental aspects and applications were described. In the following, the more recent modifications and extensions of the basic pulse sequence and their applicability to spectral assignments and structural elucidation will be briefly reviewed. Some of these more sophisticated techniques may not be as readily implementable as the basic ID TOCSY experiments, and thus have not yet found wide applications in routine practice. 

The basic pulse sequence for the NOESY experiment is just that illustrated... 

As an example, we describe one of many similar experiments devised for assigning resonances in proteins, a subject that we take up in more detail in Chapter 13. This particular experiment is designed to correlate the frequencies within the H—15N—13C=0 portion of a peptide group and is appropriately called simply HNCO. The basic pulse sequence for HNCO is shown in Fig. 12.16. To simplify the notation, instead of I, S, and T, we identify the active spins as H, N, and C, and use K to denote the spin of ar-13C. In a peptide chain, one a carbon is bonded and spin coupled to the nitrogen and another a carbon is bonded and coupled to the carbonyl carbon atom. With recombinant DNA methods, the protein is uniformly and highly enriched in both 13C and 15N, so all of these spins need be considered. 

Pulse Sequence. The basic pulse sequence for the HETCOR experiment can be considered as a derivative of the pulse sequence used for the CP experiments (Figure 46). The main difference between the CP and HETCOR pulse... 

Figure 1. Pulse sequences used to monitor the heteronuclear NOE (bottom panel) and the spin lattice relaxation (top panel). The NOE experiment is a simple extension of the basic pulse sequence introduced by Kay et al. (1989) and utilizes continuous broadband H decoupling during the preparation period to generate the NOE. Two dimensional spectra with and without H decoupling (lightly shaded region) define the NOE. The T, relaxation experiment is a simple extension of the basic pulse sequence introduced by Sklenar et al. (1987). The NOE via H decoupling rather than coherent polarization transfer is used to polarize the carbons. For both the NOE and T, measurement, the proton pulse 0 (or the delay of the corresponding reverse INEPT) is set to the magic angle as described by Palmer et al. (1991). The constant time period, A, is set to minimize cos(n [27i J + 27t Jo,]). When x is set to l/2 Jc then 2A = - 1/2 J( ...

The basic pulse sequences known from correlation have all been... 

Two-dimensional exchange spectroscopy (2D EXSY) offers an enormous extension of accessible rates towards slow and very slow exchange processes. Extending the timescale is not the only merit of 2D EXSY methods before discussing some practical examples, there should be a few general remarks. The basic pulse sequence used is shown in scheme 4. 

The decoupling efficiency of most of these RE irradiation schemes was described using AHT on non-spinning coupled spin-1/2 nuclei. Only in the case of Class IV samples was spinning taken into account during the basic pulse sequence design. 

Figure 4 (a) The generic elements of a 2D NMR experiment. The basic pulse sequences shown for (b) DQFCOSY,... 

The basic pulse sequence to generate MQ coherences consists of three pulses,... 

The pulse sequence for J-resolved spectroscopy is (90) — fj — (180). The FID is observed as an echo, so there needs to be an additional delay ty before the detection period ti, and the complete sequence is (90) — tj — (180) — tj — f2, as shown in Figure 26. For COSY the basic pulse sequence is (90) — ty— (90) — tj and for NOESY it is (90) — ty— (90) — D — (90) — 2, with a pulsed field gradient applied during the delay, D. A large number of other pulse sequences has been designed both to improve detection and to permit other forms of correlation spectroscopy. 

Figure 5.15 (A) Pulse sequence designed for the combination of polarization transfer and selective flip of attached protons. The long-range couplings are suppressed, and only one-bond heteronuclear couplings are observed in this experiment. (B) (a) Quartet of nitromethane obtained by using the basic pulse sequence of the heteronuclear 2D /-resolved experiment, (b) The same quartet, but with misset delays, which results in spurious peaks, (c) Quartet obtained by the combination of INEPT with spin manipulation. Peak intensification is due to the polarization transfer effect, (d) Effect of missetting of pulse widths in (c). (Reprinted from ]. Magn. Reson. 58, V. Rutar, 132, copyright (1984), with permission from Academic Press, Inc.)...

Eor example, a homonuclear COSY experiment (see Table 3.4) is used to map the proton-proton J coupling in a molecule. Consider a simple system in which two protons are coupled to each other, CH—CH. The basic pulse sequence for a 2D... 

A recent experiment that seems likely to assume considerable importance is unusual in not making use of coherence transfer, but rather using tj modulation to "label" longitudinal magnetizations involved in chemical exchange or cross-relaxation.The basic pulse sequence has the form ... 

Many NMR pulse sequences can be described either by a classical model describing the motions of magnetic vectors or by quantum mechanical models of different levels of sophistication. The attractive feature of the classical vector model is that it provides simple physical pictures of many of the basic pulse sequences. However, it does not work for many multipulse experiments that involve multiple... 

In NMR imaging, data acquisition pulse sequences play an important role and are intimately related to the image reconstruction algorithms employed. This description of the mathematical formulations is based on the basic pulse sequence, which uses only 90° RF pulses, although in a real imaging situation the spin-echo techniques that use additional 180° RF pulses are more common. 

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