Coherence using phase cycling

Between these pairs there will be a vertical streak (parallel to the F axis) that represents the 12C-bound proton signal. Because the 12C-bound proton signal is not modulated in t, the 13C evolution period, it has no F frequency, and so it just appears at all F frequencies that is, as a vertical streak. This problem can be solved by coherence pathway selection using phase cycling or gradients. 

Spectra obtained using magnetic field gradients to selectively filter the desired order of coherence are typically much cleaner than those obtained using phase cycling for MQ filtration, since the sequence does not rely on... 

CT pathway selection using phase cycling procedures depends on the signal accumulation of repeated scans. In each scan the same pulse sequence is executed but the phase of the excitation pulses and the receiver is shifted. The coherence selection is based on a difference method such that the unwanted coherences are cancelled because the effect of the phase shift is different for the coherence and the receiver phase. The wanted coherences are constructively added to the detected signal because the coherence and receiver have the same behaviour with respect to the phase shift. The best way to... 

A second example of coherence selection using phase cycling is the suppression of the quaternary carbon signals in a polarization transfer spectrum. DEPT and INEPT type pulse sequences use a polarization transfer step to enhance the signal of an NMR insensitive nucleus such as which exhibits scalar coupling to a NMR sensitive nucleus such as IR. 

In Check it 5.7.LI the basic structure of the ID RMQC experiment is introduced. The limits of IR coherence suppression of l C isotopomers, the so-called coherence, by phase cycling is shown and the use of the BIRD-d7 filter element as one solution to this problem is illustrated. 

Figure 7.25 Homoniiclear double-quantum filtered COSY spectrum (400 MHz) of 8-mMangiotensin II in H,0 recorded without phase cycling. Magnetic field gradient pulses have been used to select coherence transfer pathways. (Reprinted from J. Mag. Reson. 87, R. Hurd, 422, copyright (1990), with permission from Academic Press, Inc.)...

Phase cycling As employed in modern NMR experiments, repeating the pulse sequence with all the other parameters being kept constant and only the phases of the pulse (s) and the phase-sensitive detector reference being changed. The FIDs are acquired and coadded. The procedure is used to eliminate undesired coherences or artifact signals, or to produce certain desired effects (e.g., multiple-quantum filtration). 

As explained in depth in the next sections, most of the advanced high-resolution experiments on quadrupolar nuclei are based on the selection of specific coherences and on the transfer of these coherences along the selected pathways, which always terminate at the observable SQ coherence p = — 1. Most of the time, the selection is done using nested phase-cycling of the radiofrequency (rf) pulses included in the pulse sequence [34]. Recently, new methods have been proposed to optimize the nested phase-cycling procedure, including the schemes referred to as cogwheel [36-39] and multiplex [40—421. 

Technically, the inverse experiment used to be very demanding because the excess of protons not coupled to the nucleus of interest (e.g., protons coupled to the almost hundred-fold excess of 12C instead of 13C) needed to be suppressed. Originally, this was achieved by the use of elaborate phase-cycling schemes, but today the coherence pathway selection by gradient pulses facilitates this process. 

Accurate measurements of the frequency-resolved transverse spin relaxation T2) of Rb NMR on single crystals of D-RADP-x (x = 0.20, 0.25, 0.30, 0.35) have been performed in a Bq field of 7 Tesla as a function of temperature. The probe head was placed in a He gas-flow cryostat with a temperature stability of 0.1 K. To obtain the spin echo of the Rb - 1/2 -o-+ 1/2 central transition we have used the standard (90 - fi - 180y -ti echo - (2) pulse sequence with an appropriate phase-cycling scheme to ehminate quadrature detection errors and unwanted coherences due to pulse imperfections. To avoid sparking in the He gas, the RF-field Bi had to be reduced to a level where the 7T/2-pulse length T90 equalled 3.5 ps at room temperature. 

The spectral quality and the efficiency of the basic COSY and the DQ-filtered COSY experiments may be improved with the use of field gradients instead of phase cycling for coherence selection, which remove spectral artifacts and make time consuming phasecycling superfluous. 

A variety of sequences exist, which differ with respect to the detected interaction ( J, or Jx ) and the mode of detection ( C or H detected, magnitude or phased mode, phase cycling or gradients for coherence selection). In view of the reduced sensitivity of heteronudear experiments with respect to homonuclear COSY experiments and the steadily decreasing sample amounts submitted for NMR experiments, there is no doubt that the inverse ( H) detected, gradient enhanced experiments are currently the best methods to apply. However on older type spectrometers, not equipped for inverse detection the old-fashioned direct C detected experiments are still in use. 

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