Precession adjustment

The INEPT experiment can be modified to allow the antiphase magnetization to be precessed for a further time period so that it comes into phase before data acquisition. The pulse sequence for the refocused INEPT experiment (Pegg et al., 1981b) is shown in Fig. 2.13. Another delay, A. is introduced and 180° pulses applied at the center of this delay simultaneously to both the H and the C nuclei. Decoupling during data acquisition allows the carbons to be recorded as singlets. The value of Z), is adjusted to enable the desired type of carbon atoms to be recorded. Thus, with D, set at V4J, the CH carbons are recorded at VsJ, the CH2 carbons are recorded and at VeJ, all protonated carbons are recorded. With D3 at %J, the CH and CH ( carbons appear out of phase from the CH2 carbons. 

Again, there is a universal interface which adjusts the output of the precession signal generator and/or the scan generator, suitable for input into the coils of the TEM. This system is independent from the presence (or not) of STEM unit in a TEM, and can also be adapted to other types of transmission microscopes. 

The tilt angle is adjustable. Maximum half angle is between 1 and 3°, depending on the TEM coils. The period of precession is adjustable from 5 to 30 Hertz. 

A pulse width exactly adjusted to turn the double cone of precession by 90° about the x axis is called a 90° or n/2 pulse. 

Switching on the 13C RF transmitter is represented by opening the valve between the reservoirs H and 13C. The relative powers of the proton and 13C RF transmitters are adjusted to maximize interactions between the two types of precessing nuclei. Polarization can then be transferred between neighboring nuclei through spin flip-flop processes. Optimization is achieved when the Hartmann-Hahn condition is met, i.e., the H and 13C RF field strengths are in a ratio set close to 1 4 (Pines et al. 1973). Magnetization is then transferred with a time constant TCH-... 

N, or 29Si. In CP, both nuclear spin species are spin locked and the rf amplitudes adjusted so that their Larmor precession frequencies in their respective rotating frames are equal the Hartmann-Hahn condition. The single-contact CP enhancement of the rare spins is given by yj/ys. Additional enhancement normally occurs because the CP experiment may be recycled at a rate limited by the usually much faster spin-lattice relaxation rate of protons rather than that of the rare spins. 

A plot of Eq. 3.8 in the complex plane is shown in Fig. 3.3. Because the frequency of the output (a) — 0) is just the frequency at which Mxy precesses in the rotating frame, Fig. 3.3 also provides a visual depiction of this precession as viewed along the z axis. Note that the quadrature phase detection configuration permits us to differentiate between the frequencies (w — w ) and (o)rf — ), whereas such frequencies are indistinguishable with a single phase detector. The phase angle (4> — cf)r,-) can be chosen to provide the pure absorption mode on resonance, and in more complex experiments it can be adjusted as needed, as we shall see in detail later. 

Deoxy mb single crystal (A) was calibrated on precession camera (see text). The R (q>) — values are plotted in Fig. 17. °) Deoxy mb single crystal (B) was calibrated by visual adjustment only therefore we give an error for q>. 

The major difference between soft shaped pulses and DANTE methods is the occurrence of strong sideband excitation windows either side of the principal window with DANTE. These occur at offsets from the transmitter at multiples of the hard-pulse frequency, 1/x. They arise from magnetisation vectors that are far from resonance and which process full circle during the x period. Since this behaviour is precisely equivalent to no precession, they are excited as if on-resonance. Further sidebands at 2/x, 3/x and so on also occur by virtue of trajectories completing multiple full circles during x. Such multisite excitation can at times be desirable [50,51] but if only a single excitation window is required, the hard pulse repetition frequency must be adjusted by varying x to ensure the sideband excitations do not coincide with other resonances. 

Polarization is transferred between the proton and carbon nuclei as they both precess about the y -axis by adjusting the power levels of the applied fields B fj and B until the Hartmann-Hahn condition is matched (7h ih = 7c ic) The transfer of polarization is made possible because the z -com-ponents of both and magnetizations have the same time dependence... 

Figure 3.32. (A) (i) 90° pulse tips magnetization by 90° (ii) nucleus a precesses away faster than nucleus b (iii) 180° y pulse causes magnetization vectors a and b to adopt mirror image positions across the y-axis (v) nucleus a catches up with b producing a spin-echo at time 2t after the original 90° pulse. (B) If pulse angle is not adjusted correctly, the error in setting the 180° pulse causes the two new nuclei to refocus slightly above the y-axis [see (v) ]. After subsequent time period t a second 180° pulse (which is similarly maladjusted) is applied. This causes an equal and opposite error [see (vii) ] which results in production of a correctly focused spin-echo in (viii). ...

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