Basic pulse sequences

Figure Bl.13.5. Some basic pulse sequences for measurements for carbon-13 and nitrogen-15.

The basic pulse sequence for the production of a spin-echo is illustrated in Fig. 2.1. The behavior of C vectors in a heteronuclear CH sys-... 

The basic pulse sequence employed in the heteronuclear 2D shift-correlation (or HETCOR) experiment is shown in Fig. 5.40. The first 90° H pulse bends the H magnetization to the y -axis. During the subsequent evolution period this magnetization processes in the x y -plane. It may be considered to be made up of two vectors corresponding to the lower (a) and higher (/3) spin states of carbon to which H is coupled. These two... 

Figure 1.50 Basic pulse sequence for PCSE diffusion measurements. C = gradient strength, J = delay between the midpoints of the gradients,...

Fig. 1. Basic pulse sequence and CP diagram for gradient-based spin-locked ID exf>eriments. A 1 (— 1) 2 gradient ratio selects N-type data (solid lines) while 1 (— 1) (—2) selects P-type data (dashed lines). When SL stands for a -filtered DIPSI-2 pulse train, a ge-lD TOeSY is performed. On the other hand, when SL stands for a T-ROESY pulse train, a GROESY experiment is performed. S stands for the gradient length.

In the previous sections, only the basic, non-gradient ID TOCSY pulse sequence, its experimental aspects and applications were described. In the following, the more recent modifications and extensions of the basic pulse sequence and their applicability to spectral assignments and structural elucidation will be briefly reviewed. Some of these more sophisticated techniques may not be as readily implementable as the basic ID TOCSY experiments, and thus have not yet found wide applications in routine practice. 

The basic pulse sequence for the NOESY experiment is just that illustrated... 

As an example, we describe one of many similar experiments devised for assigning resonances in proteins, a subject that we take up in more detail in Chapter 13. This particular experiment is designed to correlate the frequencies within the H—15N—13C=0 portion of a peptide group and is appropriately called simply HNCO. The basic pulse sequence for HNCO is shown in Fig. 12.16. To simplify the notation, instead of I, S, and T, we identify the active spins as H, N, and C, and use K to denote the spin of ar-13C. In a peptide chain, one a carbon is bonded and spin coupled to the nitrogen and another a carbon is bonded and coupled to the carbonyl carbon atom. With recombinant DNA methods, the protein is uniformly and highly enriched in both 13C and 15N, so all of these spins need be considered. 

Choice of Pulse Sequence. Many variants of these basic pulse sequences exist. HMQC sequences are often preferred in studies of low-y metal nuclei, while HSQC sequences are preferred by authors interested in N. The present authors experience is in line with these prejudices . We have found versions incorporating adiabatic decoupling of third nuclei (eg., H, N HSQC with adiabatic decoupling of P) particularly useful (Figure 19). 

Pulse Sequence. The basic pulse sequence for the HETCOR experiment can be considered as a derivative of the pulse sequence used for the CP experiments (Figure 46). The main difference between the CP and HETCOR pulse... 

Figure 1. Pulse sequences used to monitor the heteronuclear NOE (bottom panel) and the spin lattice relaxation (top panel). The NOE experiment is a simple extension of the basic pulse sequence introduced by Kay et al. (1989) and utilizes continuous broadband H decoupling during the preparation period to generate the NOE. Two dimensional spectra with and without H decoupling (lightly shaded region) define the NOE. The T, relaxation experiment is a simple extension of the basic pulse sequence introduced by Sklenar et al. (1987). The NOE via H decoupling rather than coherent polarization transfer is used to polarize the carbons. For both the NOE and T, measurement, the proton pulse 0 (or the delay of the corresponding reverse INEPT) is set to the magic angle as described by Palmer et al. (1991). The constant time period, A, is set to minimize cos(n [27i J + 27t Jo,]). When x is set to l/2 Jc then 2A = - 1/2 J( ...

The basic pulse sequences known from correlation have all been... 

Two-dimensional exchange spectroscopy (2D EXSY) offers an enormous extension of accessible rates towards slow and very slow exchange processes. Extending the timescale is not the only merit of 2D EXSY methods before discussing some practical examples, there should be a few general remarks. The basic pulse sequence used is shown in scheme 4. 

The decoupling efficiency of most of these RE irradiation schemes was described using AHT on non-spinning coupled spin-1/2 nuclei. Only in the case of Class IV samples was spinning taken into account during the basic pulse sequence design. 

Figure 4 (a) The generic elements of a 2D NMR experiment. The basic pulse sequences shown for (b) DQFCOSY,... 

The basic pulse sequence to generate MQ coherences consists of three pulses,... 

Fig. 3. Basic pulse sequences for 2D- X,"T H correlations. Tbe same notation as in Hg. 1 is used. Minimum phase cycles for selection of correlation signals are given, more elaborate schemes for quadrature detection in FI and phase-sensitive spectra may be applied following standard rules. (a) HETCOR (without 180° pulses)/INEPT (with 180° pulses), the refocusing delays A are optional in both experiments setting the mixing pulses 8 to 45°/135° instead of 90° allows to determine coupling signs in ABX-type spectra. (b), HSQC. (c), HMQC the refocusing delay A2 is optional.

The pulse sequence for J-resolved spectroscopy is (90) — fj — (180). The FID is observed as an echo, so there needs to be an additional delay ty before the detection period ti, and the complete sequence is (90) — tj — (180) — tj — f2, as shown in Figure 26. For COSY the basic pulse sequence is (90) — ty— (90) — tj and for NOESY it is (90) — ty— (90) — D — (90) — 2, with a pulsed field gradient applied during the delay, D. A large number of other pulse sequences has been designed both to improve detection and to permit other forms of correlation spectroscopy. 

Fig. 15. Basic pulse sequence for the acquisition of MQ spectra with chemical shift refocusing.

Devise a gradient selected version of the triple-quantum filtered COSY experiment, whose basic pulse sequence and CTP was given in E9-7. Your sequence should include recommendations for the relative size of the gradients used. The resulting spectrum must have pure phase (i.e. p = 1 must be preserved in /,) and phase errors due to the evolution of offsets during the gradients must be removed. 

Figure 1 A basic pulse sequence for HSQC experiment. Narrow and wide bars represent 90 and 180 RF pulses, respectively. Pulse phase is x, if not stated otherwise.

Figure Bl.13.5. Some basic pulse sequences for measurements for carbon-13 and nitrogen-15. (Reproduced by permission of Elsevier from Kowalewski J and Maler L 1997 Methods for Structure Elucidation by High-Resolution NMR ed Gy Batta, K E Kover and Cs Szantay (Amsterdam Elsevier) pp 325-...

Figure 5.15 (A) Pulse sequence designed for the combination of polarization transfer and selective flip of attached protons. The long-range couplings are suppressed, and only one-bond heteronuclear couplings are observed in this experiment. (B) (a) Quartet of nitromethane obtained by using the basic pulse sequence of the heteronuclear 2D /-resolved experiment, (b) The same quartet, but with misset delays, which results in spurious peaks, (c) Quartet obtained by the combination of INEPT with spin manipulation. Peak intensification is due to the polarization transfer effect, (d) Effect of missetting of pulse widths in (c). (Reprinted from ]. Magn. Reson. 58, V. Rutar, 132, copyright (1984), with permission from Academic Press, Inc.)...

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