J-resolved spectra

FIGURE 10.6 Heteronuclear J-resolved l3C-H spectrum (75 MHz) of allylbutyl ether in CDC13. The ordinate F, shows the splitting from /ch i 1 Hz, and the abscissa F2 displays the, 3C chemical shifts in ppm. At the top is the completely decoupled one-dimensional, 3C spectrum. Courtesy of Herman J. C. Yeh (National Institutes of Health). 

The spin-echo pulse sequence can also be applied to produce homonuclear (e.g., H) J-resolved spectra. For such a system it is not possible to apply broadband decoupling, so the F2 dimension might be expected to display the ordinary coupled H spectrum. However, because the coupling information is independently available, it is not difficult to process the data in such a way that only the chemical shifts are displayed in the F2 dimension, as illustrated in Fig. 10.7. 

The projection on the F2 axis would show the equivalent of a completely decoupled proton spectrum. 

J-resolved spectra can be useful in disentangling or editing spectra with overlapping peaks, but other 2D experiments are generally used more frequently in the structure elucidation of organic molecules. 

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Three-dimensional NMR spectra, like 2D NMR spectra, may be broadly classified into three mtyor types (a) 3D J-resolved spectra (in which the... 

With J-Resolved spectra a further option for symmetrization is available (Symj), which compares the values (y-amplitudes) of pairs of data points situated symmetrically on opposite sides of the horizontal line through the centre of the data matrix. Again the two modes as discussed above can be selected. However in this case, the Tilt operation (section 5.4.3.2) must first be applied prior to symmetrization. 

After 2D Fourier transformation J-Resolved spectra usually contain a distortion along the horizontal line leading through the centre of the matrix. In order to get rid of this distortion and to separate chemical shifts from homonuclear J-couplings, the whole matrix is tilted. With 2D WIN-NMR a Tilt command is available which automatically adjusts the corresponding parameters (Tilt factor) and performs a tilt operation. 

A similar structural study was made with angustifoline. The H and C NMR spectra of the alkaloid in four different solvents were fully assigned by resorting to 2D H- H and COSY and 2D J resolved spectra. 

The pulse sequence, as a variant of the spin echo experiment, also refocuses the spread of frequencies caused by field inhomogeneity, so that some improvement in resolution is obtained. The inset at the lower right of Figure 6-18 shows the normal ID spectra of H-4 and H-5 at the top (Figure 6-18c and e) and the unrotated projection of the 2D J-resolved spectra at the bottom [Figure 6-18d and f, extracted from the projected spectrum (Figure 6-18a) at the top of the 2D display]. The much higher resolution of the 2D resonances is clearly evident. Thus, the procedure is an effective way to measure J accurately, particularly when J is poorly resolved in the ID spectrum. The experiment fails for closely coupled nuclei (second-order spectra). 

A similar structural study was made with angustifoline. The H and l3C NMR spectra of the alkaloid in four different solvents were fully assigned by resorting to 2D H- H and H-I3C COSY and 2D J resolved spectra. Conformational analysis in the solid state and in solution is presented [201]. More recently other studies of the conformational equilibrium of diazatricyclic systems based on one- and two- dimensional H, 3C and 15N NMR experiments were published [202,203]. 

Figure 3 F, cross-sections of the 2D C J-resolved spectra (top) and corresponding CP/MAS spectra (bottom) of (a) p-dimethoxyben/,ene 6] (b) kaolinite DMSO intercalate [14]. The two methyl quartets in the intercalate sp>ectrum come from inequivalent DMSO molecules between the kaolinite layers. Both J-resolved spectra were recorded using MREV-8 homonuclear decoupling at natural C abundance.

In addition to the importance of parameter selection on F skew, the ACCORD-HMBC experiment has also been shown to exhibit strong coupling artifact responses similar to those observed in heteronuclear 2D J-resolved spectra. - Finally, the triplet character of responses in the second frequency domain superimposed over the Fi skew was mathematically accounted for by Zangger and Armitage in their development of the ACCORD-HMQC experiment. "... 

Another way to obtain resolution of overlapping signals might be the application of the HOHAHA technique, by which spectra from the separate spin systems can be obtained, or the measurement of 2D J-resolved spectra. These techniques will be discussed below. 

J-Resolved Spectroscopy. In J-resolved spectra, the chemical shifts are displayed on one axis while on the other axis the coupling information is displayed. These spectra can be obtained both for protons and carbon-13. They can be useful in the case of crowded spectra. 

There are two broad classes of 2D-NMR spectra. In the J-resolved spectra, the peaks in a spectrum are so spread that the chemical shifts are found in one dimension while the coupling constants occur in the second dimension. The second class covers correlated spectra which are basically correlation diagrams between two spectra. Thus the H-NMR spectrum may lie along one axis while the C-NMR spectrum lies on the other axis and the contours obtained at the cross-points allow one to correlate which protons in the H-NMR spectrum are attached to which carbon atoms in the C-NMR spectrum. 

Figure 5.45. (A) The selective spin-flip method for recording heteronuclear 2D J-resolved spectra in which the signals are modulated by long-range coupling constants. The pulse sequence is shown above while its effect on the magnetization vectors is shown below it. The selective 180° pulse results in elimination of the large one-bond coupling constants, Jch ( ) j -Methylcellobioside with the torsional angles (j) and ij/ indicated.

D J-Resolved Spectra with INEPT. Another method used to obtain heteronuclear 2D J-resolved spectra utilizes polarization transfer techniques (INEPT) and is comparable to the spin-flip method. The pulse sequence used is... 

The heteronuclear 2D J-resolved spectra described earlier arose through the modulation of the signal amplitudes by the frequency of the C-H coupling constant, Jqh- In heteronuclear 2D-shift correlated spectra, on the other hand, the C signals in a compound are modulated by the Larmor frequencies (i.e., chemical shifts) of the protons to which the respective carbon atoms are bonded. 

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