Net chemical shift evolution

The chemical shift evolution (precession of spins as a result of chemical shift) can be represented as a circle with the rotation rate (in radians per second) written in the center (Fig. 7.6). This is the same as the motion of the net magnetization vector, viewed from the axis. Homonuclear product operators (Ha represented by Ia and Hb represented by Ib) undergo chemical shift evolution in the same way ... 

The pulse sequence is shown in Figure 11.48. The net twist will be zero only for the desired pathway DQC -> ZQC -> lH SQC. Of course, we have created a new problem the minimum t delay is now twice the time required for a gradient and its recovery. This will lead to a very large phase twist in F, so we can either present the data in magnitude mode, where phase is not an issue, or insert the appropriate spin echoes to refocus the chemical-shift evolution that occurs during the gradients. 

The sequence S must be anti-symmetric in time or have a net rotation axis that is stable as a function of the offset. This implies that sequence S separates the wanted and unwanted coherences using a selective coherence level change for one of both types of coherence. However S must also refocus any chemical shift evolution (which is anti-symmetric in time) as well as any offset dependent phase and tilt modulation (the net rotation axis that is stable as a function of the offset). 

Evolution (rotation of net magnetization in the x -y plane) occurs during delays, and the direction and speed of motion in the x-y plane depend on the resonant frequency of the NMR line relative to the reference frequency (v0 - A). In general, when the NMR peak is not on-resonance, there are two kinds of evolution. We think of the chemical shift as the frequency of the whole resonance or peak due to a nucleus or group of equivalent nuclei,... 

At the center of the echo all resonance offsets from interactions linear in the spin quantum number are canceled as long as these interactions operate for the full duration ofTE. Linear spin interactions include chemical shifts, heteronu-clear dipolar couplings, field inhomogeneity, field gradients, and transmitter frequency offsets but do not include quadru-polar and homonuclear dipolar couplings. There will however be a net phase evolution induced by an interaction to the extent its duration or intensity is not balanced with respect to the two halves of TE (that is, the balance with respect to amount of phase evolution on either side of the 180° pulse). 

The sequence that achieves this (Fig. 7.15) is a simple variant on the INEPT-based heteronuclear shift correlation sequence of Fig. 6.31 (HETCOR), so the loss in sensitivity is compensated somewhat by the use of a polarisation transfer step. In fact the only difference between the two lies in the net evolution of only shifts or only couplings for the whole of ti- The addition of a proton 180 pulse at the midpoint of ti here serves to refocus proton chemical shifts and heteronuclear coupling constants (so the X-spin 180° pulse of HETCOR becomes redundant) but leaves the proton homonuclear couplings free to evolve. The resulting spectrum therefore contains only proton multiplets in fi dispersed by the corresponding X-spin shifts in f2 (Fig. 7.16) ... 

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