Mixing pulse

Figure 1.45 Coherence transfer pathways in 2D NMR experiments. (A) Pathways in homonuclear 2D correlation spectroscopy. The first 90° pulse excites singlequantum coherence of order p= . The second mixing pulse of angle /3 converts the coherence into detectable magnetization (p= —1). (Bra) Coherence transfer pathways in NOESY/2D exchange spectroscopy (B b) relayed COSY (B c) doublequantum spectroscopy (B d) 2D COSY with double-quantum filter (t = 0). The pathways shown in (B a,b, and d) involve a fixed mixing interval (t ). (Reprinted from G. Bodenhausen et al, J. Magn. Resonance, 58, 370, copyright 1984, Rights and Permission Department, Academic Press Inc., 6277 Sea Harbor Drive, Orlando, Florida 32887.)...

Exchange correlation spectroscopy (E. COSY), a modified form of COSY, is useful for measuring coupling constants. The pulse sequence of the E. COSY experiment has a mixing pulse )3 of variable angle. A number of experiments with different values of /3 are recorded that eliminate the multiplet components of unconnected transitions and leave only the multiplet components for connected transitions. This simplified 2D plot can then be used to measure coupling constants. 

SECSY (spin-echo correlated spectroscopy) is a modified form of the COSY experiment. The difference in the pulse sequence of the SECSY experiment is that the acquisition is delayed by time information content of the resulting SECSY spectrum is essentially the same as that in COSY, but the mode... 

Figure 7.17 Pulse sequence for soft H,H-COSY with two selective pulses exciting the / multiplet and the second mixing pulse exciting the li multiplet of a three-spin system (/, /,)). (Reprinted from Mag. Reson. Chem. 29, H. Kessler et al., 527,...

A first pulse create.s transverse magnetization components (coherences) which evolve in the evolution period tl (DO in the schemes) with their characteristic precession frequencies (chemical shift and homonuclear J-coupling). The effect of the second (mixing) pulse is that information from one nucleus that evolves in tl is transferred to another (J-coupled) nucleus, the magentization components of which evolve and are detected in t2. Therefore, the nuclei carry information that relates not only to their own chemical shifts and coupling constants but also the corresponding information about the other, coupled spins. 

Figure 1.8. Scheme for homonuclear 2D NMR. The dotted lines indicate the bounds of the mixing period (or mixing pulse) in different experiments. D-J spectroscopy has no mixing period (Wutrich, 1986). Reproduced with permission. 

A variation of COSY called RELAY has been used to relay coherence from one coupled set of spins to a second set. RELAY employs a pulse sequence of 90°, r, 180°, t, 90° in place of the usual 90° mixing pulse. Although this method can be useful, it has largely been supplanted by TOCSY-HOHAHA, as described in Section 10.2. 

T. Vosegaard, D. Massiot and P. J. Grandinetti, Sensitivity enhancements in MQ-MAS NMR of spin-5/2 nuclei using modulated rf mixing pulses. Chem. Phys. Lett., 2000, 326, 454—460. 

Fig. 3. Basic pulse sequences for 2D- X,"T H correlations. Tbe same notation as in Hg. 1 is used. Minimum phase cycles for selection of correlation signals are given, more elaborate schemes for quadrature detection in FI and phase-sensitive spectra may be applied following standard rules. (a) HETCOR (without 180° pulses)/INEPT (with 180° pulses), the refocusing delays A are optional in both experiments setting the mixing pulses 8 to 45°/135° instead of 90° allows to determine coupling signs in ABX-type spectra. (b), HSQC. (c), HMQC the refocusing delay A2 is optional.

The phase properties of MQC transfer echoes have also been employed to selectively detect desired orders of MQ spectra. Using the basic sequence (Fig. 2), an additional (fixed) period d is inserted in the MQ pulse sequence, prior to the mixing pulse of the MQ pulse sequence (Fig. 10). MQC evolves beyond the normal tx period and coherence transfer echoes of the TV-quantum coherences maximise in the detection period at a time TVJ.35,41,42 The echoes of different coherence orders maximise at different times in the detection period and the spectra of different orders of coherence can be obtained by sampling the FID in the appropriate window in the detection period. 

A common modification of the basic COSY sequence is one in which the 90° mixing pulse is replaced with one of shorter tip angle, P, usually of 45 or 60 degrees (Fig. 5.52). These experiments are typically acquired and presented as absolute-value experiments since, strictly speaking, the use of a pulse angle less than 90° does not produce purely amplitude-modulated data. 

The use of a reduced mixing pulse largely restricts coherence transfer between transitions that are directly connected, or in other words, those... 

Figure 5.52. The COSY-p experiment. The P mixing pulse is usually set to 45...

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