Pulse sequence shift correlation spectra

Fig. 27 Dipolar/chemical shift correlated spectrum of C60-4H2O obtained by the pulse sequence shown at the top of the figure. Dipolar cross-sections are shown for the isotropic peak of benzene and C q as well as for their respective spinning sidebands 128 increments of 15 ps were used in ti, contact time was 5 ms and v =2.5 kHz. (Adopted from [76] with permission)...

Figure 15 (A) The basic HSQC (heteronuclear single quantum coherence) pulse sequence. (B) The HSQC spectrum of the aliphatic region of [1] with the C along and the H spectrum along f2- The C- H connectivities are marked for the same molecular fragment as in Figure 14. Other sequences of proto-nated carbons can be determined from the same spectrum while an />bond (n=2,3) C- H shift correlation spectrum such as HMBC, COLOC or FLOCK (see Table 2) can identify non-proto-nated carbons and tie together the molecular fragments into a complete structure.

Figure 5.65. 2D-Shift correlated spectrum of the tricyclodecane derivative, showing decoupling in the (Uj dimension through the use of the pulse sequence shown in Figure 5.64. The spectrum on the top of the diagram is the fully coupled spectrum in the right margin is the decoupled projection of the 2D spectrum. The area enclosed by the two vertical dashed lines corresponding to the multiplet for proton J contains cross peaks. Projection to the right from these cross peaks affords the various chemical shifts of protons to which proton J is coupled.

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. 

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. 10.13. 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]. 

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.

Fig. 6. Top 2D MAT sequence for correlating isotopic chemical shift and CSA with two separate experiments P+ and P . All pulses following CP are 90°. A four-step phase cycling is used with 6 = —y, x, —y, x. and 62 = —y, x, x, -y. The receiver phases are x, -x, — y, -y for the P+ pulse sequence and x, —x,y, y for the P pulse sequence. (The sign of receiver phases with an asterisk depends on the relation between the pulse phase and the receiver phase of the particular spectrometer in use. These receiver phases must be changed in sign when the quadrature phase cycle (x,y, —x, -y) of the excitation pulse and the receiver phase in a single-pulse test experiment result in a null signal.) Phase alternation of the first H 90° pulse and quadrature phase cycling of the last 13C 90° pulse can be added to the above phase cycle. The time period T can be any multiple of a rotor period except for multiples of 3. Bottom 2D isotropic chemical shift versus CSA spectrum of calcium formate powder with a three-fold MAT echo extension. (Taken from Gan and Ernst178 with permission.)...

Figure 12.12a gives a good illustration of the need for going to a third dimension to facilitate the interpretation of a crowded 2D spectrum. The NOESY spectrum of a uniformly 15N-enriched protein, staphylococcal nuclease, has so many cross peaks that interpretation is virtually impossible. However, it is possible to use, 5N chemical shifts to edit this spectrum, as indicated in Fig. 12.121) and c in a three-dimensional experiment. With the 15N enrichment, NOESY can be combined with a heteronuclear correlation experiment, in this case HMQC, but HSQC could also be used. A 3D pulse sequence can be obtained from two separate 2D experiments by deleting the detection period of one experiment and the preparation period of the other to obtain two evolution periods (q and t2) and one detection period (f3). In principle, the two 2D components can be placed in either order. For the NOESY-HMQC experiment, either order works well, but in some instances coherence transfer proceeds more efficiendy with a particular arrangement of the component experiments. We look first at the NOESY-HMQC sequence, for which a pulse sequence is given in Fig. 12.13. The three types of spins are designated I and S (as usual), both of which are H in the current example, and T, which is 15N in this case.

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