Modulated Spin-Echo

13p 90S 1 180 1 FID Fig. 2.39. Pulse sequence of the /-modulated spin-echo experiment [50 a]. In SEFT experiments [50 b], the proton decoupler is switched off during the second half of the evolution 

41 and 2.42(a) make clear how the switch-off delay z of the proton decoupler has to be adjusted in order to sort CH multiplets For z = (27) 1, only quaternary carbon atoms give rise to intense positive signals (Fig. 2.42(c)). For r = (7) 1, non-protonated and CFI2 carbon nuclei appear positive in contrast to negative CH and CH3 

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APT Attached proton test, a modification of the J-modulated spin-echo experiment to determine C//multiplicities, a less sensitive alternative to DEPT... 

Fig. 2.42. I3C NMR spectra of D-camphor in tetradeuteriomelhanol at 15.08 MHz (a). /-modulation of aliphatic carbon signals depending on the decoupling delay z, a verification of Fig. 2.41 (b) proton broadband decoupled spectrum (c-e). /-modulated spin-echo experiments with z = 4, 6, and 8 ms for CH multiplicity analysis (f-g) spectra with off-resonance (0 and gated decoupling of protons (g) for comparison.

Fig. 2.47. 13C NMR spectra of anhydro-panaxadiol (15 tng/mL deuteriochloro-form 100.6 MHz (a-e) 400 scans (f) 1600 scans) (a) proton broadband decoupled (f) proton off-resonance decoupled (b) subspectrum of quaternary carbon atoms resulting from a /-modulated spin-echo experiment (c-e) edited CH(I subspectra obtained from DEPT experiments with 0f = 45c, 90° and 135° [57],...

Preparation, evolution and detection are the time periods of pulse sequences described for. /-modulated spin-echo and polarization transfer experiments. The basic FT NMR technique operates without any evolution period tt immediately after generation of transverse magnetization (preparation) its free induction decay S(t2) is detected. Subsequent Fourier transformation provides the FT NMR spectrum S (f5). 

A constant evolution period t, is the new feature of. /-modulated spin-echo, DEPT, and INADEQUATE sequences. During this time period, a 7-modulation or a polarization transfer may evolve. Such pulse sequences provide FID signals S(t2) which are still functions of one variable time t2. The Fourier transforms, however, are NMR spectra with specific information, depending on the constant evolution period ti. One simple example is the generation of the quaternary carbon-13 subspectrum by means of a. /-modulated spin-echo experiment with an evolution time of tj2 = x = as in... 

It is a varying evolution time t, which brings the second dimension into the NMR experiment, provided this second variable time f, induces periodic changes of the signal amplitude or the signal phase. In the 7-modulated spin-echo experiment (Fig. 2.42(a)), for example, the switch-off delay x = -2 of the proton decoupler can be increased stepwise by a constant increment At (e.g. 1 s in Fig. 2.42(a)) in a series of k subsequent single experiments. Thereby, the FID signals will become functions of two variable times, t, and... 

Fig. 2.51.. /-Modulated spin-echo sequence with gated proton decoupling for acquisition of -/-resolved two-dimensional 13C NMR spectra, and the CH magnetization vectors in the x y plane controlled by pulses and. /-modulation. During the preparation period between successive experiments, nuclear Overhauser enhancement of 13C magnetization is retained by minimum proton decoupling.

Since JCH coupling is only in operation during one t,/2 period in the J-modulated spin-echo sequence with gated proton decoupling, as drawn in Fig. 2.51, the second Fourier transformation provides only one half of the actual Jal magnitudes. Therefore, resolution of smaller couplings will be poor. 

Modern techniques.. /-Modulated spin-echo ( APT ), non-selective polarization transfer ( DEPT ) experiments and two-dimensional carbon-proton, proton-proton ( COSY ) and carbon-carbon ( 2D-INADEQUATE ) shift correlations are in use (see the following section) and will be used in order to clarify doubtful signal assignments (see Sections 2.9 and 2.10). 

Fig. 5.1. C NMR spectra of 5a-cholcstan-3-onc in dcutcriochloroform (50 mg/0.5 mL) (a) proton broadband-decoupled, 400 scans (b). /-modulated spin-echo experiment for quaternary carbon selection, 1000 scans (c-e) CH, Cl I2, and CH3 subspectra generated from linear combination of three DEPT experiments (see Section 2.9.3.2), 200 scans per experiment (f) gated proton-decoupled experiment for comparison.

Fig. 5.1 demonstrates for cholestane-3-one the utility of recording proton broadband-decoupled spectra, performing. /-modulated spin echo experiments, and generating subspectra from linear combinations of DEPT experiments for the unequivocal identification of primary, secondary, tertiary and quaternary carbon atoms. 

The signals of a large number of coumarin derivatives [418] were assigned using the single-frequency off-resonance decoupling,. /-modulated spin echoe and INEPT techniques. 

This used to be a common way of distinguishing methyl, methylene and methine carbons. However, as can be seen in the spectrum above, it is not a clean experiment methylene carbons with non-equivalent protons attached often give particularly messy results ( 8.2.2). /-modulated spin-echoes, INEPT or DEPT provide much more reliable ways of determining multiplicities ( 3.3.2 and 8.5). 

While setting up for the previous spectra, we obtained the spectrum opposite. The spectrum has been phased to correct the phase of the highest field methyl signal. Individual signals are expanded across the spectrum so that their phases can be seen more clearly the phase errors cannot be removed. Such phase problems may be generated in several ways, but in this case one parameter was misset in the /-modulated spin-echo sequence. What is the most likely source of this problem ... 

Let us investigate what happens when we alter the normal, 3C ( H experiment in Figure 12.2 by using the /-modulated spin echo pulse sequence shown in Figure 12.8a. This pulse sequence is closely related to the spin echo technique described in Section 3.6.3. 

EXAMPLE 12.11 How will the appearance of a quaternary carbon signal obtained using the /-modulated spin echo technique vary with x ... 

EXAMPLE 12.12 Draw the. /-modulated spin echo 13C spectrum of 2-chlorobutane (Example 12.6) that results from using x = 8 ms. 

FIG. 8. View of the transverse magnetization projected on to the xy-plane in the rotating frame for a heteronuclear AX spin system. The pulse at time f = 0 is applied along the x-direction to tip the magnetization from the z- to the y-axis, and the pulses at time t = t are applied along the y-direction. (a) Formation of unmodulated spin-echo, (b) Formation of modulated spin-echo. Adapted from ref. 145. 

We now consider two special cases in which the diffusion spectra are nontrivial and Eq. (66) may be used to evaluate the result of a modulated spin-echo experiment. The first case concerns slow molecular collision rates. As pointed out by Einstein (1956), the result... 

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