Pulse sequence correlation spectra

The pulse sequence which is used to record CH COSY Involves the H- C polarisation transfer which is the basis of the DEPT sequence and which Increases the sensitivity by a factor of up to four. Consequently, a CH COSY experiment does not require any more sample than a H broadband decoupled C NMR spectrum. The result is a two-dimensional CH correlation, in which the C shift is mapped on to the abscissa and the H shift is mapped on to the ordinate (or vice versa). The C and //shifts of the //and C nuclei which are bonded to one another are read as coordinates of the cross signal as shown in the CH COSY stacked plot (Fig. 2.14b) and the associated contour plots of the a-plnene (Fig. 2.14a and c). To evaluate them, one need only read off the coordinates of the correlation signals. In Fig. 2.14c, for example, the protons with shifts Sh= 1.16 (proton A) and 2.34 (proton B of an AB system) are bonded to the C atom at c = 31.5. Formula 1 shows all of the C//connectivities (C//bonds) of a-pinene which can be read from Fig. 2.14. 

SECSY (spin-echo correlated spectroscopy) is a modified form of the COSY experiment. The difference in the pulse sequence of the SECSY experiment is that the acquisition is delayed by time mixing pulse, while the mixing pulse in the SECSY sequence is placed in the middle of the period. The information content of the resulting SECSY spectrum is essentially the same as that in COSY, but the mode... 

In 2003, Sprang and Bigler have developed a pulse sequence, HMBC-RELAY, subsequently improved in 2004, that yields two simultaneously detected types of long-range correlation spectra.60,61 One spectrum shows all 7ch connectivities while the other shows exclusively 2/ch connectivities. Their method uses homonuclear 3/hh couplings between the protons of adjacent carbons, as already been exploited for the XCORFE... 

The potential advantages of the IMPACT-HMBC experiment are readily shown in Figures 34 and 35. In Figure 34, the full-width HMBC spectrum, recorded using the standard HMBC pulse sequence, is shown on the left. This spectrum combines strong residual a/CH correlations and F, ridges... 

Fig. 2 (a) DRAMA pulse sequence (using % = t/2 = rr/4 in the text) and a representative calculated dipolar recoupled frequency domain spectrum (reproduced from [23] with permission), (b) RFDR pulse sequence inserted as mixing block in a 2D 13C-13C chemical shift correlation experiment, along with an experimental spectrum of 13C-labeled alanine (reproduced from [24] with permission), (c) Rotational resonance inversion sequence along with an n = 3 rotational resonance differential dephasing curve for 13C-labeled alanine (reproduced from [21] with permission), (d) Double-quantum HORROR experiment along with a 2D HORROR nutation spectrum of 13C2-2,3-L-alanine (reproduced from [26] with permission)... 

Fig. 10 (a) Rf trajectory, (b) pulse sequence, (c) offset (upper) and rf inhomogeneity (lower) profile for EXPORT (c subpanels c, e, f) relative to DCP (c subpanels a, d) and optimal control DCP (c subpanel b), along with a 2D 13C-13C correlation spectrum of GB1 obtained using EXPORT with 23.8 kHz spinning and no 1H decoupling (reproduced from [48] with permission)... 

Fig. 10.13. 2D J-resolved NMR spectrum of santonin (4). The data were acquired using the pulse sequence shown in Fig. 10.12. Chemical shifts are sorted along the F2 axis with heteronuclear coupling constant information displayed orthogonally in F . Coupling constants are scaled as J/2, since they evolve only during the second half of the evolution period, t /2. 13C signals are amplitude modulated during the evolution period as opposed to being phase modulated as in other 13C-detected heteronuclear shift correlation experiments.

Using strychnine (1) as a model compound, a pair of HSQC spectra are shown in Fig. 10.16. The top panel shows the HSQC spectrum of strychnine without multiplicity editing. All resonances have positive phase. The pulse sequence used is that shown in Fig. 10.15 with the pulse sequence operator enclosed in the box eliminated. In contrast, the multiplicity-edited variant of the experiment is shown in the bottom panel. The pulse sequence operator is comprised of a pair of 180° pulses simultaneously applied to both H and 13C. These pulses are flanked by the delays, A = l/2(xJcii), which invert the magnetization for the methylene signals (red contours in Fig. 10.16B), while leaving methine and methyl resonances (positive phase, black contours) unaffected. Other less commonly used direct heteronuclear shift correlation experiments have been described in the literature [47]. 

Figure 10.1 shows a two-dimensional [15N, H]-TROSY correlation spectrum of the 15N,2H- labeled 110 kDa homo-octameric protein 7,8-dihydroneopterin aldolase from Staphylococcus aureus (DHNA) measured with the pulse sequence of Fig. 10.4 [13]. The gain in spectral resolution and sensitivity is readily apparent from comparison with the corresponding conventional experiment. The optimal sensitivity is achieved by adjusting the polarization transfer r in Fig. 10.4 (3 ms <2r<5.4 ms [3]). For an optimal suppression of the non-TROSY components, the so-called Clean TROSY might be used [19]. Similar signal and spectral resolution enhancements are achieved for 15N,2H-labeled or 13C,15N,2H-...

Fig. 18. The pulse sequence for measuring the 2D heteronuclear correlation NMR spectrum with frequency-switched Lee-Goldburg irradiation during the evolution.

Fig. 3. Sections of two-dimensional 31P/15N H correlation spectra of the azido-substituted monophosphazene derivative shown. The 2D spectra were recorded by using a conventional 31P/15N HMQC pulse scheme with phase-cycling (left), and the gradient-enhanced enhanced sensitivity HSQC pulse sequence of Fig. 2 (right). The onedimensional spectra on top of the 2D-maps were acquired with the lD-versions of both pulse sequences. The right spectrum is distinguished by a substantially lower artefact level and displays an additional clearly visible correlation of the 31P with nitrogen atom N3. Reproduced from Ref. 25 by permission of Elsevier Ltd.

Fig. 13. (a) 1H/(31P)/15N correlation of a mixture of Mes P( = NH) = NMes (compd. 2, Mes = 2,4,6-tri-t-butylphenyl) and Mes P(NHMes )-N1 = N2 = N3 (compd. 3) with correlations involving the iVH and aromatic protons in the P-Mes substituents. The spectrum was obtained with the pulse sequence shown in Fig. 11a. The tx noise around S1H = 5.1 is due to a solvent signal (CH2C12) which is 4 105 times more intense than that of the 15N-satellites of the iVH-resonance of 3. (b) Expansion of a -detected 2D-/P N-resolved spectrum of the same mixture with correlations of the aromatic protons in the P-Mes -substituents as obtained with the pulse sequence shown in Fig. 12. 2q cross-sections of the 2D-spectrum at the chemical shifts of the aromatic protons of 2 and 3 are given in (c) and (d), respectively, and reveal the presence of one (2) and three (3) resolved JP N couplings. Reproduced from Ref. 43 by permission of John Wiley Sons.

HSQC rather than HMQC-based transfer schemes have recently in particular been employed in various indirectly detected two- and three-dimensional 111/X/Y correlation experiments involving multi-step coherence-transfer in either direction.38 40 43 44 The application of PFG s appears to be essential to obtain a sufficiently clean spectrum that is free of artefacts, and in many cases the pulse sequence shows only a satisfactory performance if composite pulses, with a larger excitation bandwidth than normal ones, are employed.21,38,39,43 The pulse schemes yield generally phase-sensitive spectra with pure absorptive lines and do not suffer from splitting or broadening of the cross peaks as a consequence of the undesired evolution... 

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