Complex Pulse Sequences

Assume for the moment that all three coupling constants in our AMX system are positive, and focus your attention on the A and M multiplets. Lines A and A2 correspond to the up orientation of nucleus X, while lines A3 and A4 correspond to the down orientation of X. Similarly, lines M, and M2 correspond to the up orientation of nucleus X, while lines M3 and M4 correspond to the down orientation. We describe this situation by saying that the A -A2 doublet is connected (via the similar spin state of nucleus X) with the M,-M2 doublet, and the A3-A4 doublet is connected with the M3-M4 doublet. These connections are shown below the spectrum. If we simultaneously saturate only lines A and A2 (by focusing v2 between them and carefully adjusting its power), only the connected lines (doublet Mj-M2) will collapse to a singlet. Lines M3 and M4 would remain intact. 

Without doubt the biggest advances in NMR technology over the past decade have involved the advent of higher field pulse-mode spectrometers together and the realization that, by careful manipulation of the Vj and v2 pulse sequences, the net magnetization vector M can be redirected at will to 

There are two things to note at this point. First, the 180,. pulse inverts M (rotates it 180° around the x axis) to the -z axis, giving an excess of spins.in the higher energy spin state (a population inversion). The 180y- pulse inverts M around they axis. Such a pulse will become important in the next section.  

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The INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) experiment [6, 7] was the first broadband pulsed experiment for polarization transfer between heteronuclei, and has been extensively used for sensitivity enhancement and for spectral editing. For spectral editing purposes in carbon-13 NMR, more recent experiments such as DEPT, SEMUT [8] and their various enhancements [9] are usually preferable, but because of its brevity and simplicity INEPT remains the method of choice for many applications in sensitivity enhancement, and as a building block in complex pulse sequences with multiple polarization transfer steps. The potential utility of INEPT in inverse mode experiments, in which polarization is transferred from a low magnetogyric ratio nucleus to protons, was recognized quite early [10]. The principal advantage of polarization transfer over methods such as heteronuclear spin echo difference spectroscopy is the scope it offers for presaturation of the unwanted proton signals, which allows clean spec-... 

Isotopes in low abundance have long spin-lattice relaxation times which give rise to poor signal-to-noise ratios. Sensitivity can be improved by using a technique known as cross polarization where a complex pulse sequence transfers polarization from an abundant nucleus to the dilute spin thereby enhancing the intensity of its signal. 

DOUBLE-RESONANCE TECHNIQUES AND COMPLEX PULSE SEQUENCES... 

There is a large and growing class of complex pulse sequences that involve polarization transfer from a set of more sensitive nuclei (e.g., 1H) to a set of less sensitive nuclei (e.g., I3C), greatly increasing the signal intensity of the latter set. Examples include the following ... 

Basic NMR Experiments—A Practical Course by S. Braun et al,41 describes the operation of an NMR spectrometer and, as its title implies, gives guidance, with specific experimental parameters, for carrying out a variety of NMR procedures—from measuring the width of a 90° pulse to complex pulse sequences in two- and three-dimensional NMR. 

Summation of the results of the four repetitions gives a signal for path A but none for path B. This approach can be used for more complex pulse sequences to devise effective phase cycles. 

The use of an additional 90° pulse with timing and phase cycling as given here acts as a low-pass filter, because it eliminates the high frequency component from the larger j/. This technique is also used in other complex pulse sequences as a filter. 

In practice, HNCO is now carried out by a somewhat more complex pulse sequence than that given in Fig. 12.16 in order to improve its efficiency. Pulsed field gradients are added to aid coherence pathway selection an INEPT transfer from N to K replaces the multiple quantum coherence step and the N evolution is carried out with a constant time experiment. 

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