Multiple quantum coherence applications

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.

In order to carry out complete structural elucidation of unknown compounds (especially for complex molecules), the RF probe should enable a variety of heteronuclear NMR techniques to be performed. In particular, inverse detection H-15N and 1H-13C experiments such as heteronuclear multiple quantum coherence (HMQC) [29,30] and heteronuclear single quantum coherence (HSQC) [31] find almost ubiquitous application in myriad research environments. Although the microliter-scale probes described above feature both heteronuclear and homonuclear capabilities, no commerical product is... 

One of the more recent applications of proton NMR in solid polymers has been the use of multiple quantum coherence (MQCOH), which is developed among clusters of dipolar coupled protons vide infra) to infer details of chain dynamics [20], and (which is another way of making the same statement) local dipolar couplings [21]. 

The DEPT pulse sequence is illustrated in Fig. 4.31. To follow events during this, consider once more a H- C pair and note the action of the two 180 pulses is again to refocus chemical shifts where necessary. The sequence begins in a similar manner to INEPT with a 90 (H) pulse after which proton magnetisation evolves under the influence of proton-carbon coupling such that after a period 1 /2J the two vectors of the proton satellites are antiphase. The application of a 90 (C) pulse at this point produces a new state of affairs that has not been previously encountered, in which both transverse proton and carbon magnetisation evolve coherently. This new state is termed heteronuclear multiple quantum coherence (hmqc) which, in general, cannot be visualised with the vector model, and without recourse to mathematical formalisms it is... 

The DQF-COSY sequence (Fig. 5.40) differs from the basic COSY experiment by the addition of a third pulse and the use of a modified phase-cycle or gradient sequence to provide the desired selection. Thus, following tj frequency labelling, the second 90° pulse generates multiple-quantum coherence which is not observed in the COSY-90 sequence since it remains invisible to the detector. This may, however, be reconverted into single-quantum coherence by the application of the third pulse, and hence subsequently detected. The required phase-cycle or gradient combination selects only signals that existed as double-quantum coherence between the last two pulses, whilst all other routes are cancelled, hence the term double-quantum filtered COSY. 

The other common inverse-detection method, heteronuclear multiple quan-turn coherence (HMQC) relies on multiple-quantum coherence transitions during the pulse sequence. Due to the multiple-quantum coherence transitions it is more laborious to theoretically follow the course of magnetization, and the cross peak will be broader in the Fi dimension due to the /hh evolution. Unlike HSQC, HMQC can also be optimized for Jch couplings. This heteronuclear multiple bond correlation experiment, or HMBC, ° ° has lower sensitivity than HMQC/HSQC experiments, and the Jch correlations can appear as artefacts in the spectrum. However, the cross peak volume should follow the concentration of analyte, so with proper method validation HMQC and HMBC should also be applicable for quantification. 

Fig. 8.19 Schematic representation of the gradient heteronuclear multiple quantum coherence or GHMQC pulse sequence. The gradient version of this experiment now in use [114] is derived from the earlier non-gradient experiment described by Bax and Subramanian [113]. Coherence pathway selection is obtained by the application of gradients in a ratio of 2 2 1 as shown. Other ratios are also possible, as considered in the reports of Ruiz-Cabello et al. [115] and Parella [116]. The experiment creates heteronuclear multiple quantum coherence with the 90° C pulse that precedes evolution. Both zero and double quantum coherences are created and begin to evolve through the first half...

Normally, the experimentally observed longitudinal and transverse relaxations involve zero- and single-quantum processes Am = i — y = 0 or 1). The other time-dependent density matrix elements p (t) with A i > 1 are classified as multiple-quantum coherences. Although these coherences are not directly observable, it is possible to measure every element of the relaxation density matrix by employing specially designed NMR methods. These coherences are very important for multiple-quantum experiments [13] and quantum computation NMR applications, as it will be described later. 

Double-or multiple-quantum coherence cannot be detected as such but must be converted into single-quantum coherence (or observable magnetization in the -plane) by the application of a selective 180° pulse to the X spin. This results in the introduction of a phase shift of the signals which allows one to distinguish the pairs of doublets due to couplings from the undesired... 

Of the two isotopes of nitrogen, N with a spin quantum number of 1/2 is the most widely used. It is very insensitive (1 x 10" ), and has a low natural abundance of 0.37%. While some studies have used the direct detection of N-labelled compounds, the indirect detection of N through H by techniques such as INEPT (insensitive nuclei enhanced by polarization transfer) and HMQC (heteronuclear multiple quantum coherence), which considerably enhance sensitivity, have increased the applicability of N in biological systems. 

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