Pulse NMR experiments

The result is that the net magnetization along the y axis decays to 0 as the spin isochromats fan out in x -y plane. 

As mentioned in the previous section, the time constant which describes the decay of the magnetization in the x-y plane is T. We define an intrinsic relaxation time which is characteristic of the magnetization decay in one of the spin 

Ingeneous experiments have been designed to remove the effect of the applied field inhomogeneity. They are called spin-echo techniques and have application in the realm of 

One way to increase the efficiency of data taking in such a case is a sequence called DEFT for driven equilibrium Fourier transform (Becker, et al. 1969). The idea of this experiment is to force the system back to equilibrium more rapidly after each FID so that it can be pulsed more frequently. In a DEFT sequence, a 90° pulse is applied to the system, and the FID is recorded. A 180° pulse is applied at a time x when the FID has disappeared due to the dephasing of the spin isochromats to form an echo at time 2x. At the 

We shall now return to spin echoes by expanding the discussion to include echo trains. For simplicity, let us assume that we have only one line in a high resolution spectrum. On exact resonance in an inhomogeneous field, the 90° pulse yields an FID with a time constant T. At a time x later, a 180° pulse is applied and the echo maximum occurs at time 2x, since the time required for rephasing the spin iso- 

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Many other pulsed NMR experiments are possible, and some are listed in the final sections. Most can be canied out using the standard equipment described above, but some require additions such as highly controllable, pulsed field gradients, shaped RF pulses for (for example) single-frequency irradiations, and the combined use of pulses at several different frequencies. 

Spin-spin relaxation is the steady decay of transverse magnetisation (phase coherence of nuclear spins) produced by the NMR excitation where there is perfect homogeneity of the magnetic field. It is evident in the shape of the FID (/fee induction decay), as the exponential decay to zero of the transverse magnetisation produced in the pulsed NMR experiment. The Fourier transformation of the FID signal (time domain) gives the FT NMR spectrum (frequency domain, Fig. 1.7). 

Summarize the different events during a pulsed NMR experiment,... 

Gated decoupling The decoupler is gated during certain pulse NMR experiments, so spin decoupling occurs only when the decoupler is switched on and not when it is switched off used to eliminate either H- C spincoupling or nuclear Overhauser effect in a ID C spectrum, and employed as a standard technique in many other H-NMR experiments, such as APT and y-resolved. 

NMR provides one of the most powerful techniques for identification of unknown compounds based on high-resolution proton spectra (chemical shift type integration relative numbers) or 13C information (number of nonequivalent carbon atoms types of carbon number of protons at each C atom). Structural information may be obtained in subsequent steps from chemical shifts in single-pulse NMR experiments, homo- and heteronuclear spin-spin connectivities and corresponding coupling constants, from relaxation data such as NOEs, 7) s 7is, or from even more sophisticated 2D techniques. In most cases the presence of a NOE enhancement is all that is required to establish the stereochemistry at a particular centre [167]. For a proper description of the microstructure of a macromolecule NMR spectroscopy has now overtaken IR spectroscopy as the analytical tool in general use. 

For this purpose, standard 5-mm NMR tubes were charged with 100 pL ethynylbenzene, 6 mg of the catalyst Pdx[N(octyl)4Cl]y, and 0.7 mL acetone-dg and placed into a 200-MHz spectrometer. Charges of 51%-enriched p-H2 were prepared as previously outlined via catalytic equilibration over charcoal at 77 K and injected repeatedly in synchronization with the pulsed NMR-experiment via an electromechanically lowered glass capillary mechanism. 

In a basic pulsed NMR experiment (for I = 1/2), when a sample is placed in the applied magnetic field (B0), the nuclear spins distribute themselves between parallel and antiparallel positions, according to Boltzmann distribution [Eq. (11)] (Figure 21 A). The number of spins in the parallel position is slightly greater than that in the antiparallel position. At equilibrium, the spins are processing randomly (i.e., lack phase coherence). The populations... 

In a pulse NMR experiment, the z magnetization is flipped into the xy plane, and the individual transitions start to process. During the detection,... 

Shriver, J., Product Operators and Coherence Transfer in Multiple-Pulse NMR Experiments, Concepts in Magnetic Resonance An Educational Journal, 1992,4 (No 1)... 

The main problem now is to justify the process.schematically depicted by Reactions 11 and 12. From pulse NMR experiments, Mestdagh, Stone, and Fripiat (12) have measured the lifetime (r) of a proton on a surface oxygen site in a decationated Y zeolite... 

In most pulsed NMR experiments, the rf field is applied off-resonance. Modulated pulse interferograms (Fig. 2.4(e), 2.5(a), 2.6(a), and 2.7(a)) arise because the vectors of transverse magnetization do not precess with a constant phase shift of itj2 relative to the vector Bj. This is demonstrated in Fig. 2.8. The transverse magnetization is then a resultant of two components, t>(f) with a phase shift of n/2 relative to B, and u(r) in phase with Bt ... 

Pulsed NMR experiments enable one to determine separately the homonuclear, M2, and the heteronuclear, M2S, components of the second moment... 

Notice how we diagram a multiple-pulse NMR experiment the horizontal axis represents time and the vertical axis represents RF amplitude for pulses. The times and amplitudes are not drawn to scale—they are just cartoon representations. 90° pulses are shown as half the width of 180° pulses, and recording of the FID is shown as a decaying signal. Each RF channel is labeled according to the nucleus being irradiated (pulses) and/or observed (FID). 

Shriver J. Product operators and coherence transfer in multiple-pulse NMR experiments. Concepts Magn. Reson. 1992 4 1-33. 

Fig. 5.4.4a-c. nC NMR sequences a Routine sequence with broad band H decoupling, b Inverse gated decoupling sequence (1GD) for quantitative analysis c Time sequence for 1-D and 2-D pulse NMR experiments... 

The ratios Po Jpoa and PBalPwr and hence K, are to be determined from the relative integrated peak areas of the deuteron and proton NMR spectra. A sensitive Fourier-transform NMR instrument with multinuclei capability is thus required, ideally at 200-MHz proton frequency or higher. Here we outline the essentials of a pulsed NMR experiment more detailed discussions can be found in Refs. 4 to 7. 

In a pulsed NMR experiment, the smiple is irradiated with a sdong, wide bmd radiofrequency pulse. As a result of this, all die nuclei process widi die Larmor frequencies characteristic of dieir environments generating a dansverse magnetic field which decays exponentially. This is called/ree induction decay and contains all die information of a conventional frequency spectnun except that it requires conversion by Fourier transformation using a computer into... 

These operators induce transitions between different spin states so that by applying an rf-field to nuclear spins in the presence of a large static magnetic field close to the Larmor frequency, the spin distribution between the energy levels is perturbed away from thermodynamic equilibrium. In pulsed NMR experiments the spin system is excited with a short if pulse near resonance and the system is measured afterwards. The total external Hamiltonian from the applied fields is then... 

Due to the low protein content of normal human CSF, it is possible to use single-pulse NMR experiments to obtain biochemical information without recourse to the spin-echo techniques often required for studies on blood and plasma (see Section 5). However, where serious cerebral damage has occurred, or in the presence of an acute infection, spectra may become dominated by protein resonances. Problems may also arise from protein binding and metal binding of some metabolites, with a consequential broadening of their proton resonances. 

In order to understand how proton NMR of polymers may be used as presented in the current chapter, it is useful to remind ourselves of some of the results of the formalism used in describing the time-dependent quantum mechanics of spin which are not so well understood by those familiar with only the rudimentary pulse NMR experiment. We do so in Section 6.1.2. In Section 6.1.3, examples are given of the uses of the mechanics outlined in Section 6.1.2 to provide information about chemical functionality, motion... 

Figure 2.6. The laboratory and rotating frame representations. In the laboratory frame the co-ordinate system is viewed as being static, whereas in the rotating frame it rotates at a rate equal to the applied rf frequency, vq. In this representation the motion of one component of the applied rf is frozen whereas the other is far from the resonance condition and may be ignored. This provides a simplified model for the description of pulsed NMR experiments.

Figure 4.1. The essential elements of the single-pulse NMR experiment the relaxation (recovery) delay, the pulse excitation angle and the data acquisition time.

The INEPT experiment [26] (Insensitive Nuclei Enhanced by Polarisation Transfer) was one of the forerunners of many of the pulse NMR experiments developed over subsequent years and still constitutes a feature of some of the most widely used multidimensional experiments in modem pulse NMR. Its purpose is to enable non-selective polarisation transfer between spins, and its operation may be readily understood with reference to the vector model. Most often it is the proton that is used as the source nucleus and these discussion will relate to XH spin systems throughout, although it should be remembered that any high-y spin- /2 nucleus constitutes a suitable source. 

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