Heteronuclear shift-correlation pulse sequence

Figure 13. Heteronuclear chemical shift correlation pulse sequence used with the phase cycling scheme [14] in acquiring the spectrum of Figure 12.

Figure 5.40 (A) Pulse sequence for the 2D heteronuclear shift-correlation experiment. (B) Effect of the pulse sequence in (A) on H magnetization vectors of CH.

The SELINCOR experiment is a selective ID inverse heteronuclear shift-correlation experiment i.e., ID H,C-COSYinverse experiment) (Berger, 1989). The last C pulse of the HMQC experiment is in this case substituted by a selective 90° Gaussian pulse. Thus the soft pulse is used for coherence transfer and not for excitation at the beginning of the sequence, as is usual for other pulse sequences. The BIRD pulse and the A-i delay are optimized to suppress protons bound to nuclei As is adjusted to correspond to the direct H,C couplings. The soft pulse at the end of the pulse sequence (Fig. 7.8) serves to transfer the heteronuclear double-quantum coherence into the antiphase magnetization of the protons attached to the selectively excited C nuclei. 

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.

Fig. 10.14. Gradient-enhanced HMQC pulse sequence described in 1991 by Hurd and John derived from the earlier non-gradient experiment of Bax and Subramanian. For 1H-13C heteronuclear shift correlation, the gradient ratio, G1 G2 G3 should be 2 2 1 or a comparable ratio. The pulses sequence creates heteronuclear multiple quantum of orders zero and two with the application of the 90° 13C pulse. The multiple quantum coherence evolves during the first half of ti. The 180° proton pulse midway through the evolution period decouples proton chemical shift evolution and interchanges the zero and double quantum coherence terms. Antiphase proton magnetization is created by the second 90° 13C pulse that is refocused during the interval A prior to detection and the application of broadband X-decoupling.

Fig. 10.15. Pulse sequence for the multiplicity-edited gradient HSQC experiment. Heteronuclear single quantum coherence is created by the first INEPT step within the pulse sequence, followed by the evolution period, t . Following evolution, the heteronuclear single quantum coherence is reconverted to observable proton magnetization by the reverse INEPT step. The simultaneous 180° XH and 13C pulses flanked by the delays, A = l/2( 1 edits magnetization inverting signals for methylene resonances, while leaving methine and methyl signals with positive phase (Fig. 16A). Eliminating this pulse sequence element affords a heteronuclear shift correlation experiment in which all resonances have the same phase (Fig. 16B).

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]. 

Hyphenated heteronuclear shift correlation methods Many of the experiments described thus far can be used as building blocks to create more sophisticated pulse sequences. The earliest variants of hyphenated NMR experiments, such as HC-RELAY, were heteronuclear detected [56]. More recent hyphenated heteronuclear shift correlation methods are based on proton detection and include... 

The development of carbon-13 NMR during the last eight years has been characterized by a continual increase in the sensitivity and quality of spectra. A reduction in measuring time - equivalent to an enhancement in sensitivity has been achieved mainly by cryomagnet technology. The efficiency with which NMR information can be obtained has been substantially improved by new computer-controllable pulse sequences for one-and two-dimensional NMR experiments. A selection of these new methods, in particular, those used for multiplicity analysis and homo- or heteronuclear shift correlations, is presented in chapter 2 of this edition. 

The sequence that achieves this (Fig. 7.15) is a simple variant on the INEPT-based heteronuclear shift correlation sequence of Fig. 6.31 (HETCOR), so the loss in sensitivity is compensated somewhat by the use of a polarisation transfer step. In fact the only difference between the two lies in the net evolution of only shifts or only couplings for the whole of ti- The addition of a proton 180 pulse at the midpoint of ti here serves to refocus proton chemical shifts and heteronuclear coupling constants (so the X-spin 180° pulse of HETCOR becomes redundant) but leaves the proton homonuclear couplings free to evolve. The resulting spectrum therefore contains only proton multiplets in fi dispersed by the corresponding X-spin shifts in f2 (Fig. 7.16) ... 

Figure 5.40 (A) Pulse sequence for the 2D heteronuclear shift-correlation experi-...

The first proton-detected long-range heteronuclear shift correlation experiment to be developed was the HMBC experiment reported in 1986 by Bax and Summers." The pulse sequence for the experiment is shown in Fig. 5. The experiment is quite simple in constitution, utilizing only a total of five pulses. Following the first proton pulse, a fixed delay, A, is in.serted, optimized as a function of ( Jch)- The phase of the following 90° C or X-pulse is alternated 0202 while the receiver phase is alternated 0022. This pulse operator is referred... 

Fig. 8.28 The (/, -HMBC experiment is the most sophisticated accordion-optimized long-range heteronuclear shift correlation experiment reported to date [148]. The experiment uses a pulse sequence operator known as a STAR (selectively tailored F, accordion refocusing) to selectively manipulate two-bond and three-bond long-range correlations to protonated carbon or nitrogen resonances. A. STAR operator used in the (/, J-HMBC experiment. The experiment takes advantage of the ability of a BIRD(x,x,x) pulse to refocus the one-bond heteronuclear coupling of a protonated carbon. By doing this, the coupling to this proton...

NMR has been a powerful technique for structural analyses of macromolecules. However, ID NMR spectra of PDMS are usually complicated due to signal overly. Their complete characterization often requires combinations of several techniques. Multidimensional NMR techniques, especially inversely detected 3D heteronuclear shift correlation experiments, offer the opportunity to obtain the complete structural characterization by using NMR experiments alone. Biological 3D-NMR experiments are usually performed in conjunction with uniform and isotopic labeling. In polymer chemistry, when isotopic labeling is possible, it is often very difficult and expensive. By modifying the 3D-pulse sequence used for biopolymers, triple resonance 3D-NMR techniques have been adapted for sbufying the structures of polymers, which involve H- C- P, H- C- Si spin... 

LI Semiselective Refocused Heteronuclear Shift Correlation Spec-troscopy. Semiselective refocusing has recently been employed in heteronuclear spectroscopy for detection of proton-decoupled proton spectra. This allows detection of heteronuclear couplings without complications from proton-proton couplings, and affords spectra with increased sensitivity and resolution. The pulse sequence used is 90°( H)-t-180°( H)-180°(X)-t-90°( H) where X is the heteronucleus. This allows selective refocusing of only those protons which are directly coupled to the heteronucleus. 

Figure 5.57. Pulse sequences for heteronuclear shift correlation experiment (A) normal mode (B) employing homonuclear broad-band decoupling (COLOC).

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