J-Modulated Spin Echo

Difficulties are encountered in establishing carbon multiplicities with the SFORD technique when signal overlap becomes severe, or when only small quan-titites of sample are available, thus dividing the signal intensity between the lines of the multiplet. New techniques that overcome the limitations of SFORD utilizing the spin echo are the J-modulated spin-echo (231) or the Attached Proton Test (312), the Insensitive Nuclei Enhanced by Polarization Transfer (56, 79, 104), and the Distortionless Enhancement by Polarization Transfer (80). 

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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... 

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. 

The J-modulated spin-echo technique " and the DEPT technique " are pulse sequences, which transform the information of the CH signal multiplicity and of spin-spin coupling into phase relationships (positive and negative amplitudes) of the C signals in the proton decoupled C NMR spectra. The DEPT technique benefits from a polarisation transfer which increases the sensitivity by up to a factor of 4. For... 

Figure 4.13. J-modulated spin-echo sequences, (a) The decoupler-gated variant and (b) the pulsed variant.

Figure 4.15. The variation of carbon signal intensities in the J-modulated spin-echo as a function of the evolution time A (0 = ISOJA ).

Figure 4.16. Carbon spectrum of camphor 4.1 edited with the J-modulated spin-echo sequence, (a) Conventional carbon spectrum (carbonyl not shown), and edited spectra with (b) A = 1/J (6 = 180°) and (c) A = 1/2J (0 = 90°) with J assumed to be 130 Hz. Some breakthrough of protonated carbons is observed in (c) due to variations in coupling constants within the molecule.

The poor editing accuracy of spin-echoes in the presence of a wide range of J values and the inability to fully characterise all carbon multiplicities are the major limitations of these techniques. More complex variations on the pulsed J-modulated spin-echo are to be found that do allow a complete decomposition of the carbon spectra into C, CH, CH2 and CH3 sub-spectra [21] and which also show greater tolerances to variations in Jch [22]. Likewise,... 

Spectral Editing Using J-Modulated Spin-Echos... 

The combination of these two spectra then allows direct identification of XH,XH2 and XH3 signals in even the most complex molecules in a similar manner as in the J modulated spin echo experiments described above. 

While the SPT experiment has obvious utility, it is cumbersome to use unless the selective nature of the experiment is specifically being exploited for structure elucidation purposes. A group of experiments that may be categorized as J-modulated spin echo experiments allows the simultaneous investigation of the entire spectrum. As the name of this group of experiments implies, they utilize a spin echo of the type shown in Eq. (1), over which a scalar coupling driven process (using jcH most commonly) is superimposed. 

Fig. 8.6 Pulse sequence for the amplitude modulated 2DJ-resolved NMR experiment. The experiment is based on a J-modulated spin echo. The first 90° pulse tips magnetization into the xy-plane where it evolves during the first half of the evolution period, t]/2. The 180° pulse is applied and the decoupler is simultaneously gated off for the second half of evolution.

Figure 23. J-modulated spin-echo pulse sequence...

Another technique for determining carbon multiplicities similar to the J-modulated spin echo, but with added enhancement due to a polarization transfer from the protons to nuclei, is INEPT (Insensitive Nuclei Enhanced by Polarization Transfer) (56, 79, 104). The polarization transfer is capable of a roughly four-fold enhancement of the carbon signal intensity. 

The J-modulated spin-echo method described here is of particular advantage when the signals lie close to one another, and determination of... 

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