Multiple pulses, phase-coherent, coherence

B. Coherence Control by Phase-Coherent Multiple Pulses... 

Multiple Phase-Coherent Laser Pulses in Optical Spectroscopy. I. The Technique and Experimental Applications, W. S. Warren and A. H. Zewail, J. Chem. Phys. 78, 2279 (1983). 

Fig. 8.19. Vector representation of a H-13C HMQC experiment. The first 90° pulse along y rotates the equilibrium magnetization of the proton spin, /H, from the z axis to the x axis. After a time /d = 1/2/Hx, the antiphase coherence 2/J1/ t (see Appendix IX) is at its maximum. A 90° pulse on carbon along y then transforms the antiphase coherence into a MQ (multiple quantum) coherence (the 2/J1/ component is shown). During t the MQ evolves (with a 180 refocusing pulse on proton in the middle), until a further 90 pulse on carbon along x transforms the —2/ / component (shown at its maximum for clarity) into a 2/ /f antiphase coherence. After the time fd, in-phase coherence of the proton spin develops. The latter is detected during h. Its initial intensity is modulated by the carbon Larmor frequency during t (if proton refocusing has been used), thus originating a proton-carbon cross peak.

Both limitations can be avoided if tailor-made multiple-pulse sequences are used for band-selective Hartmann-Hahn transfer. The so-called tailored TOCSY sequences TT-1 and TT-2 (see Table 4) were the first crafted band-selective Hartmann-Hahn sequences to be reported in the literature (Glaser and Drobny, 1989). Both phase-alternated sequences do not use any supercycling scheme. The TT-1 sequence with vf = 10 kHz was developed for band-selective coherence transfer between the offset ranges R- (-2.5 kHz < < —1.5 kHz) and Rj (1.5 kHz < Vj < 2.5 kHz). 

In this sequence the first pulse creates multiple-quantum (MQ) coherences from proton antiphase magnetization having evolved under a /( C, H) coupling, while the second C pulse creates such MQ coherences for the magnetization arising from couplings. Adequate phase cycling... 

The overall result is that anti-phase magnetization of spin 1 has been transferred into anti-phase magnetization of spin 2. Such a process is called coherence transfer and is exceptionally important in multiple-pulse NMR. 

With respect to the pulse sequence layout, the HMBC experiment is essentially a HMQC experiment incorporating a low-pass filter to suppress the one-bond correlation peaks. The low-pass filter, consists of a delay d2 = 1/(2 U(C, H)) and a 90° pulse, which transfers the U(C, H) coherence into a multiple quantum state. In a second period coherences which are generated by JCC, H) evolution are also transferred to a multiple quantum state by a 90° l C pulse but with a different phase in relation to the first 90° pulse of the low-pass filter. A combination of appropriate receiver phase cycling and pulse phase cycling enables the exclusive detection of J(C, H) correlation peaks in the 2D experiment. 

The technique employs a specially designed 2D NMR pulse sequence [ 100], which forces nuclear spins to act collectively via their dipolar couplings, thereby creating unobservable multiple-quantum (MQ) coherences. The MQ coherences then evolve in the /, time domain and, after conversion into observable singlequantum coherences, are indirectly detected over the acquisition time t2- The multiple-quantum information is contained in the F dimension of the 2D spectrum (Fig. 13). The modified phase-incremented experiment [ 105] proceeds with the evolution held fixed and offers ease of operation and more accurate intensity distributions in return for loss of information contained in the fine structure and shapes of the MQ peaks. 

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