Gradient-selected spectroscopy

The term traditional in this context refers to those experiments that make use of phase cycling to select the desired signals and suppress all others. The notion of phase cycling has already been encountered in previous sections the point to recall at this stage is that this procedure involves the repetition of a pulse sequence with the phases of the if pulse(s) adjusted 

---

Fig. 5.5.8 The volume selective spectroscopy rf pulse sequence used to acquire the data shown in Figures 5.5.9-5.5.11. Magnetic field gradients applied alongthex, y and zdirections enable localized spectra to be recorded from...

A wide variety of ID and wD NMR techniques are available. In many applications of ID NMR spectroscopy, the modification of the spin Hamiltonian plays an essential role. Standard techniques are double resonance for spin decoupling, multipulse techniques, pulsed-field gradients, selective pulsing, sample spinning, etc. Manipulation of the Hamiltonian requires an external perturbation of the system, which may either be time-independent or time-dependent. Time-independent... 

Fig. 9 A volume-selective spectroscopy r.f. pulse sequence. Magnetic field gradients applied along x-, y-, and z-directions enable recording of localized spectra from pre-defined local volume elements within the sample.

Fig. 2. Pulse scheme for the gradient-selected, sensitivity-enhanced X/Y se-HSQC experiment as employed for 31P/15N correlation spectroscopy in Ref. 25. 90° and 180° hard pulses are denoted by solid and open bars, respectively. 2 are delays of length 1/(4 /x,y)> and is a short delay of the same length as the gradient pulse (typically rj 1 ms). Pulse phases are x, unless specified. The ratio of gradient pulse strengths is set to G2/Gi = Yy/Yx, and quadrature detection in Fi is achieved by recording every transient twice and changing the sign of G2 in the second scan.

Fig. 9.1.4 [Rom2] Pulse and gradient scheme for lock-pulse selective spectroscopy. The slice selection is based on selective spin locking.

This scheme can be repeated for different gradient directions. Without repetition a slice is selected, with one repetition a line and two repetitions a voxel (Fig. 5.3). Thus, selective excitation is needed to prepare individual volume elements in an extended object for subsequent investigation by NMR spectroscopy [19j. This type of NMR with spatial resolution is called volume-selective spectroscopy. 

WiUker, W., Leibfritz, D., Kerssebaum, R., and Bermel, W. (1992) Gradient selection in inverse heteronuclear correlation spectroscopy. Magn. Reson. Chem. 31, 287-292. 

Figure 8.39. ID NOESY schemes. The basic scheme (a) requires difference spectroscopy to reveal the NOEs whereas the gradient-selected DPFGSE-NOE (b) reveals NOEs without interference from difference artefacts.

NMR has become a standard tool for structure determination and, in particular, for these of Strychnos alkaloids. The last general article in this field was authored by J. Sapi and G. Massiot in 1994 [65] and described the advances in spectroscopic methods applied to these molecules. More recently, strychnine (1) has even been used to illustrate newly introduced experiments [66]. We comment, here, on their advantages and sum up the principles of usual 2D experiments in Fig. (1) and Fig. (2) (COSY Correlation SpectroscopY, TOCSY TOtal Correlation SpectroscopY, NOESY Nuclear Overhauser Enhancement SpectroscopY, ROESY Rotating frame Overhauser Enhancement SpectroscopY, HMQC Heteronuclear Multiple Quantum Coherrence, HMBC Heteronuclear Multiple Bond Correlation). This section updates two areas of research in the field new H and 13C NMR experiments with gradient selection or/and selective pulses, 15N NMR, and microspectroscopy. To take these data into account, another section comments on the structure elucidation of new compounds isolated from Strychnos. It covers the literature from 1994 to early 2000. 

The development of NMR experiments with gradient selection has proven to be of fundamental importance for homo- and heteronuclear 2D applications. One of the main problems encountered in classical 2D spectroscopy is the differentiation between wanted and unwanted coherences. 

FIGURE 3.9 The first of three plots of the same region of a H- H 2-D gradient-selected correlation spectroscopy (gCOSY) spectrum showing the effect of plot threshold variation. In this plot, the plot threshold Is set too high (too little Information appears In the 2-D plot). 

The homonudear 2-D NMR experiments that use I-coupling indude the correlation spectroscopy (COSY, and variants induding gradient-selected COSY or gCOSY, double-quantum filtered COSY or DQF-COSY) experiment, the total correlation spectroscopy (TOCSY) experiment, and the incredible natural abimdance double quantum transfer experiment (INADEQUATE) [3]. 

Further modifications (the addition of a spin-echo) allow the direct and relayed peaks to be differentiated through inversion of the direct correlations [38], and gradient-selected versions of these sequences without [39, 40] and with [10,41] such editing have also been proposed. Although the addition of TOCSY transfer is probably the most useful extension, any homonuclear mixing scheme can, in principle, be added, including a COSY, NOESY (NOE spectroscopy) or ROESY (ROE spectroscopy) step. 

Tolman, J. R., Chung, J. and Prestegard, J. H. Pure-phase heteronuclear multiple-quantum spectroscopy using field gradient selection. J. Magn. Reson. 98 462—467, 1992. 

---