Coherence pathway

Figure Bl.12.12. Pulse sequences used in multiple quantum MAS experiments and their coherence pathways for (a) two-pulse, (b) z-filter, (c) split-t with z-filter and (d) RIACT (II).

Bain A D 1984 Coherence levels and coherence pathways in NMR. A simple way to design phase cycling procedures J. Magn. Reson. 56 418-27... 

Before the application of a pulse, only equilibrium magnetization exists, directed toward the z-axis corresponding to the zero coherence level for all coherence pathways. When the pulse is applied, two coherence levels, + 1 and —1, are created during the evolution period that evolve into M and respectively, where [Pg.74]

Apparently, all coherence pathways will therefore start at zero coherence levels and end at -t-1 coherence levels since the quadrature receiver is sensitive only to the +1 polarization, only the single-quantum coherence is detected. 

Coherence pathway selection Choosing a corresponding phase cycle so that undesired resonances are suppressed and only the desired magnetization is observed. 

They also showed that a spin-echo segment with two it pulses improves the echo signal due to the inclusion of a stimulated-echo coherence pathway. 

When Tcp is longer in a CPMG sequence under a constant gradient, the diffusion effect can be observed. For the Flahn echo coherence pathway where every jt pulse refocuses the dephasing, the diffusion contribution is,... 

In 2000, Gan proposed the previously mentioned STMAS method, which is similar to MQMAS in that it can be performed on a standard MAS probehead, but uses a different coherence pathway [141]. The STMAS protocol, schematically described in Fig. 10, relies on excitation of the SQ ST coherences of order q (with q yk 0 and p = —1), instead of symmetric MQ coherences with q = 0. Note that such excitation inevitably involves the CT, as well. The ST coherences are converted to the CT coherence at t = r/(l + k) using suitable rf-pulse pulse(s). 

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. 

In the CBCA(CO)NH experiment the coherence pathway includes the carbonyl spin, which suppresses the intra-residual coherence transfer. However, as for the other two pairs of pulse sequences mentioned above, an experiment can be designed that allows simultaneous intra- and inter-residual coherence transfer. In the CBCANH experiment... 

Figure 2 Pulse sequences and coherence pathway diagrams for the phase-modulated split-t) (A) STMAS and (B) triple-quantum MAS NMR, showing the generic sequences with the specific values for Mg being r=24/31, r —O, r"=7/31 and s=12/31, s =0, s"— 9/3 and (C) numerical simulations of the relative sensitivity of the ST and MQ experiments as a function of the ratio between the RF field strength and the quadrupolar coupling...

To improve the spectral quality the unwanted H signals of the C isotopomers may be further reduced by using field gradients which will allow the proper selection of the coherence pathways in conjunction with the HMBC experiment... 

Kingsley, P.B., Product Operators, Coherence Pathways and Phase Cycling, Concepts in Magnetic Resonance An Educational Journal, 1995, 7 (No 1,2 and 3)... 

COHERENCE ORDER USING GRADIENTS TO SELECT A COHERENCE PATHWAY... 

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