Spin pulse

An alternative method to get rid of the biexponentiality of the transverse I spin relaxation is to use the basic experimental scheme known as the measurement of Ti in the rotating frame or instead of the CPMG approach. This was proposed, for the purpose of carbon-13 studies, by Ohuchi et al. [46] already in the late seventies. The general relation between T2, and other related quantities was discussed in that work and in the important paper by Vega [47]. The basic idea of the Tip measurements is illustrated in fig. 4(b). After the initial (7t/2) I-spin pulse, the phase of the radiofrequency field is switched by 90°. The transverse mag-... 

This is as in the previous experiment, but an additional delay d2 is included after the last S spin pulse (p9) to refocus IS spin coupling and allow GARP decoupling. ... 

In practice the subtraction would be carried out by shifting the receiver by 180°, so the I spin pulse would go y, -y and the receiver phase go x, -x. This is a two step phase cycle which is probably best viewed as difference spectroscopy. 

The S spin coherence order only changes when pulses are applied to those spins. The first 90° S spin pulse generates ps = 1, just as before. As by this point pI = +1, the resulting coherences have ps = +1, pt = -1 (heteronuclear zero-quantum) and ps = +1, pj = +1 (heteronuclear double-quantum). The I spin... 

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. 

It is evident from the figure that impurities can complicate the use of NMR integrals for quantitation. Further complications arise if the relevant spins are not at Boltzmaim equilibrium before the FID is acquired. This may occur either because the pulses are repeated too rapidly, or because some other energy input is present, such as decoupling. Both of these problems can be eliminated by careful timing of the energy inputs, if strictly accurate integrals are required. 

More generally, note that the applieation of almost any multiple pulse sequenee, where at least two pulses are separated by a time eomparable to the reeiproeal of the eoupling eonstants present, will lead to exehanges of intensity between multiplets. These exehanges are the physieal method by whieh eoupled spins are eorrelated in 2D NMR methods sueh as eorrelation speetroseopy (COSY) [21]. 

The sinc fiinction describes the best possible case, with often a much stronger frequency dependence of power output delivered at the probe-head. (It should be noted here that other excitation schemes are possible such as adiabatic passage [9] and stochastic excitation [fO] but these are only infrequently applied.) The excitation/recording of the NMR signal is further complicated as the pulse is then fed into the probe circuit which itself has a frequency response. As a result, a broad line will not only experience non-unifonn irradiation but also the intensity detected per spin at different frequency offsets will depend on this probe response, which depends on the quality factor (0. The quality factor is a measure of the sharpness of the resonance of the probe circuit and one definition is the resonance frequency/haltwidth of the resonance response of the circuit (also = a L/R where L is the inductance and R is the probe resistance). Flence, the width of the frequency response decreases as Q increases so that, typically, for a 2 of 100, the haltwidth of the frequency response at 100 MFIz is about 1 MFIz. Flence, direct FT-piilse observation of broad spectral lines becomes impractical with pulse teclmiques for linewidths greater than 200 kFIz. For a great majority of... 

For quadnipolar nuclei, the dependence of the pulse response on Vq/v has led to the development of quadnipolar nutation, which is a two-dimensional (2D) NMR experiment. The principle of 2D experiments is that a series of FIDs are acquired as a fimction of a second time parameter (e.g. here the pulse lengdi applied). A double Fourier transfomiation can then be carried out to give a 2D data set (FI, F2). For quadnipolar nuclei while the pulse is on the experiment is effectively being carried out at low field with the spin states detemiined by the quadnipolar interaction. In the limits Vq v the pulse response lies at v and... 

Figure Bl.12.9. Pulse sequence used for CP between two spins (/ S).

The interval between the second and third pulse is called the mixing time, during which the spins evolve according to the multiple-spin version of equation B 1.13.2 and equation B 1.13.3 and the NOE builds up. The final pulse converts the longitudinal magnetizations, present at the end of the mixing time, into detectable transverse components. The detection of the FID is followed by a recycle delay, during which the equilibrium... 

Figure Bl.14.1. Spin warp spin-echo imaging pulse sequence. A spin echo is refocused by a non-selective 180° pulse. A slice is selected perpendicular to the z-direction. To frequency-encode the v-coordinate the echo SE is acquired in the presence of the readout gradient. Phase-encoding of the > -dimension is achieved by incrementmg the gradient pulse G...

Closer examination of equation B 1,14,3 reveals that, after the slice selection pulse, the spin isocln-omats at different positions in the gradient direction are not in phase. Rather they are rotated by i exp jyC. )tind... 

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