Measuring coupling constants pulse sequences

Exchange correlation spectroscopy (E. COSY), a modified form of COSY, is useful for measuring coupling constants. The pulse sequence of the E. COSY experiment has a mixing pulse )3 of variable angle. A number of experiments with different values of /3 are recorded that eliminate the multiplet components of unconnected transitions and leave only the multiplet components for connected transitions. This simplified 2D plot can then be used to measure coupling constants. 

Although developed to measure T2, this pulse sequence is able to improve resolution or eliminate coupling constants or chemical shifts after a single cycle. Moreover, it may be modified to achieve other effects. To obtain information about how many protons are attached to a carbon, the coupling information must be manipulated in a fashion different from that used, for example, in Figure 5-5. This is a double-resonance procedure, with pulses... 

G. W. Vuister, M. Tessari, Y. Karimi-Nejad and B. Whitehead, Pulse Sequences for Measuring Coupling Constants , p. 195... 

All disilanes and trisilanes were synthesized with methods described in the literature [6]. 29Si NMR spectra were recorded with a BRUKER MSL 300 spectrometer, using solutions of the silanes in C6D6 (app. 50 %). 29Si29Si coupling constants were measured with the standard INADEQUATE pulse sequence. 

Figure 34 Excerpts of two-dimensional HMBC spectra of cholesteryl acetate recorded on a Bruker Avancell 400 MHz spectrometer (A) with the standard HMBC pulse sequence (Figure 1), and (B) with the IMPACT-HMBC experiment depicted in Figure 30. The same contour levels are used for all spectra. In (A), F, ridges are still visible (indicated by a vertical arrow), while they are very efficiently suppressed in (B). The proposed sequence results in signals with no coupling structure, as a result of the incorporation of a constant-time period. The improved peak dispersion is shown for the correlation between C-3 and H-2 (expanded in the small boxes). Asterix and the dashed box indicate residual Vch signals. The measurement duration was 22 min for both experiments.

A sequence suitable for measurement of J(H, P) and J(C, P) couplings is shown in Fig. 7.9a. The pulse sequence is a constant-time [13C, H]-HSQC (heteronuclear single-quantum correlation), in which 31P decoupling is applied in ot, in the first experiment and in co2 in the second. 

Conceptually similar to FIDS is the so-called /-modulated CT-HSQC experiment [14] (Fig. 7.8b). The coupling evolves in one experiment during x and in a second experiment not at all. The intensity ratio between first and second experiment is cos (njx). As an example we show the measurement of an NH dipolar coupling by this method. The pulse sequence of the constant time HSQC and the oscillatory behavior of the cross-peak intensities are shown in Fig. 7.11. 

The method relies on the measurement of cross-correlated relaxation rates in a constant time period such that the cross-correlated relaxation rate evolves during a fixed time r. In order to resolve the cross-correlated relaxation rate, however, the couplings need to evolve during an evolution time, e.g. tt. The first pulse sequence published for the measurement of the cross-correlated relaxation rate between the HNn and the Ca j,Ha i vector relied on an HN(CO)CA experiment, in which the Ca chemical shift evolution period was replaced by evolution of 15N,13C double and zero quantum coherences (Fig. 7.20). 

Fig. 11.7 a Pulse sequence for rotational-resonance recoupling of homonuclear spin pairs, b The spinning frequency is matched to the isotropic chemical-shift difference, and one of the resonances is selectively inverted and the polarization exchange measured as a function of the mixing time, c The difference polarization as a function of the mixing can be evaluated to give the dipolar coupling constant. 

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