Pulse NMR

Consider the 90° pulse. Immediately following it, the magnetization lies along the rotating y axis. This magneti- 

Because the pickup coil is sensitive only to the component of magnetization in the x-y plane, the magnetization being re-established along the z axis due to the Tj process is not 

The effect of a 90° pulse can be duplicated in cw NMR although with more difficulty than in pulse NMR. When a resonance line is swept rapidly in the conventional cw high resolution experiment, a ringing pattern follows the line if phase detection is used. Such a ringing pattern occurs in the following way As one sweeps into resonance, the mag- 

The magnetization decay rate constant in the x-y plane, 1/T, consists of a term related to the recovery rate constant for the magnetization along z, 1/Tj, as well as other mechanisms which give rise to a dispersal of the magnetization only in the x-y plane. Two examples of the latter are 1/T which results from dipolar processes (to be discussed later) and YAHq which is due to the applied magnetic field inhomogeneity AHq. In this case the relaxation rate is approximately 

The development of the NMR hardware and the improvements in protein expression and labelling led to the situations when the duration of a multidimensional experiment is governed not by the sensitivity, but by the resolution in each dimension. The intrinsic properties of digital FT require uniform independent sampling of each indirect dimension, leading to the dramatic increase of the total 

Nuclear Magnetic Resonance, Volume 34 The Royal Society of Chemistry, 2005 

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As in the case of infrared, progress in computing and the development of powerful algorithms for Fourier transforms has made the development of pulse NMR possible. 

Although continuous wave NMR is sufficient for naturally abundant nuclei with strong magnetic moments such as hydrogen, fluorine and phosphorous, the study of low abundance nuclei and/or weak magnetic moments such as carbon 13 or silicon 29 requires pulse NMR. 

Carbon 12, the most abundant naturally occurring isotope, has zero spin and thus cannot be studied by NMR. On the other hand, its isotope carbon 13 has an extra neutron and can be its low natural occurrence (1.1%) nevertheless makes the task somewhat difficult. Only pulsed NMR can be utilized. 

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. 

Fukushima E and Roeder S B W 1981 Experimental Pulse NMR (Reading, MA Addison-Wesiey)... 

Bain A D 1988 The superspin formalism for pulse NMR Prog. Nucl. Magn. Reson. Spectrosc. 20 295-315... 

E. Eukushima and S. B. W. Roeder, Experimental Pulse NMRs ANuts andBolts Approach, Addison-Wesley Pubhshing Co., Inc., Reading, Mass., 1981. 

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). 

The Fourier transform of a pure Lorentzian line shape, such as the function equation (4-60b), is a simple exponential function of time, the rate constant being l/Tj. This is the basis of relaxation time measurements by pulse NMR. There is one more critical piece of information, which is that in the NMR spectrometer only magnetization in the xy plane is detected. Experimental design for both Ti and T2 measurements must accommodate to this requirement. 

Pseudo-affine deformation scheme 96, 97 Pseudohexagonal (rotator) phase 67 Pseudorotator phase 67 Pulsed NMR techniques 30... 

It is curious that the chair- boat problem, which is most associated with small, liquid-state molecules, arises in the context of solid-state research (B3, II). Although the paucity of useful experiments militates against a definitive solution here E3), the frequency independence of the NMR second moment (E2), the absence of an observable free-induc-tion decay (Tj <25 fis) in the pulsed NMR spectrum (El), and the smoothness of the absorption mode itself (SI), all argue against the... 

Tanaka H. and Nishi T., Study of block copolymer interface by pulsed NMR, J. Chem. Phys., 82, 4326, 1985. 

The new interface model and the concept for the carbon black reinforcement proposed by the author fundamentally combine the structure of the carbon gel (bound mbber) with the mechanical behavior of the filled system, based on the stress analysis (FEM). As shown in Figure 18.6, the new model has a double-layer stmcture of bound rubber, consisting of the inner polymer layer of the glassy state (glassy hard or GH layer) and the outer polymer layer (sticky hard or SH layer). Molecular motion is strictly constrained in the GH layer and considerably constrained in the SH layer compared with unfilled rubber vulcanizate. Figure 18.7 is the more detailed representation to show molecular packing in both layers according to their molecular mobility estimated from the pulsed-NMR measurement. 

This conclusion is supported by the experimental result " given by the pulsed-NMR measurement that the spin-spin relaxation time T2 is considerably shorter for the gel than that for the matrix mbber vulcanizate, which of course, indicates that the modulus is considerably higher for the gel than for the matrix mbber. More quantitatively, Maebayashi et al. measured the acoustic velocity of carbon gel by acoustic analysis and concluded that the compression modulus of the gel is about twice that of matrix mbber. Thus, at present, we can conclude that the SH layer, of course without cross-linking, is about two times harder than matrix cross-linked mbber in the filled system. 

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

In pulse NMR we measure in the time domain i.e., the variation of signal amplitude with time (FID) is recorded. These time-domain data are then subjected to Fourier transformation to convert them into the frequency domain. 

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. 

Fig. 40.14. Effect of zero filling on the back transform of the pulse NMR signal given in Fig. 40.12. (a) before zero filling, (b) after zero filling.

E. Fukushima, S.B.W. Roeder 1986, Experimental Pulse NMR - a Nuts and Bolts Approach, 10lh edn, Perseus Publishing, Cambridge, MA, 556 pp. Standard textbook on basics of NMR, focus not on applications. 

E. Fukushima, S. B. W. Roeder 1981, Experimental Pulse NMR A Nuts and Bolts Approach, Addison-Wesley, Reading, MA. 

K. J. Packer, C. Rees 1972, (Pulsed NMR studies of restricted diffusion. 1. Droplet size distributions in emulsions),/. Colloid Interface Sci. 40, 206. 

P. J. Barker, H. J. Stronks 1990, (Application of the low resolution pulsed NMR MINISPEC to analytical problems in the food and agriculture industries), in NMR Applications in Biopolymers, eds. 

M. C. M. Gribnau 1992, (Determination of solid/liquid ratios of fats and oils by low-resolution pulsed NMR), Trends Food Sci. Technol. 3, 186-190. 

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. 

On freezing and subsequent thawing, an aqueous solution of PVA changes to a PVA gel [24], According to X-ray diffraction and pulsed NMR studies [25,26], the gelation of PVA solution is based upon microcrystallization of PVA chains as crosslinked domains. When the freezing and thawing process is... 

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