Decoupling sequence quantum coherence

An alternative way of acquiring the data is to observe the signal. These experiments are referred to as reverse- or inverse-detected experiments, in particular the inverse HETCOR experiment is referred to as a heteronuclear multiple quantum coherence (HMQC) spectmm. The ampHtude of the H nuclei is modulated by the coupled frequencies of the C nuclei in the evolution time. The principal difficulty with this experiment is that the C nuclei must be decoupled from the H spectmm. Techniques used to do this are called GARP and WALTZ sequences. The information is the same as that of the standard HETCOR except that the F and F axes have been switched. The obvious advantage to this experiment is the significant increase in sensitivity that occurs by observing H rather than C. 

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.

It is not necessary that the evolving 13C coherences be detected immediately. As shown in Section 9.6, they can be allowed to precess until they are in phase, then detected while protons are decoupled to provide a single enhanced signal. Alternatively, the entire INEPT sequence can be treated as the preparation period of a 2D experiment. The coherences then evolve during a period t, and can be manipulated in various ways by further pulses. One of the most commonly used methods is to apply a second INEPT sequence, without the initial 90v pulse, after the evolution period to convert the 13C coherences back into H coherences, which can be observed. As we mentioned in Chapter 10, this method, heteronuclear single quantum coherence (HSQC), is widely employed to obtain... 

BIRD-HMQC. The most difficult aspect of implementing the HMQC experiment is the suppression of signals from protons attached to C (the center-band or single quantum coherences) in favor of the protons attached to C (the satellites or double quantum coherences). The use of pulse field gradients (PFG, Section 6-6) is the most effective technique, but relatively few spectrometers are equipped with the hardware required for their generation. Fortunately, there is an effective alternative for the suppression of center bands by means of the BIRD Bilinear Rotation Decoupling) sequence, which is outlined by the vector... 

A product operator analysis of the observable coherences for the standard DEPT sequence [5.59] shows that in addition to the wanted coherence Sx (where Sx is the x-component of the S spin product operator and S is the non-abundant 3c spin) several other single quantum coherences are generated. These terms such as I Sx lead to phase distortions in the H coupled spectra because signals which could be assigned to either the Sx or Sy state are overlapped by antiphase signals from coherences like I Sy. Consequently the signal intensity of the related decoupled spectrum is reduced by these coherences. In the DEPT++ sequence only the required single quantum coherences Sx (or... 

Check it 5.2.6.16 also illustrates that the PENDANT+ sequence should be used for coupled spectra of CH groups which exhibit homonuclear J(H, H) coupling. In contrast to the PENDANT+ experiment, under these conditions the conventional PENDANT spectrum displays considerable lineshape distortion that cannot be corrected. These lineshape distortions can be traced back to underlying antiphase zero quantum coherences of the order I Sx or I Sy (I = H, S = l c) which are transferred by the IH purge pulse of the improved sequence to undetectable multiple quantum coherences. Broadband IH decoupling during acquisition destroys this antiphase coherence such that only very minor differences are apparent when comparing the decoupled versions of the experiment. 

Fig. 6. Pulse sequence for the refocused- or D-HMBC experiment. The idea of refocusing anti-phase proton single quantum coherence prior to acquisition to allow broadband heteronuclear decoupling during acquisition was first reported by Bermel et al in 1989. Evidently unaware of the initial report, Furihata and Seto again de.scribed this experiment in a 199.5 communication, giving it the acronym D-HMBC. There have been a number of applications of this experiment for the acquisition of both H- Cand long-range...

Fig. 5. HMBC pulse sequence of Bax and Summers (1986). The first 90° pulse serves as a low-pass J-filter (Kogler et al. 1983), as discussed in the text. Heteronuclear multiple-quantum coherence of order zero and two is created by the second 90° pulse. The 180° H pulse interchanges zero- and double-quantum coherences and decouples proton chemical shift evolution during tp Observable proton single-quantum coherence is recreated by the final 90° pulse and detected...

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