Flip angle experiments

Many subspectral editing techniques alternative to DEPT, such as SEMUT (Subspectral Editing using a Multiple Quantum Trap) (Bildsoe et al., 1983) and SEMUT GL, have been developed that utilize the fact that the transfer of magnetization to unobservable multiple-quantum coherence for CH, CHj, and CH spin systems is dependent on the last flip angle 0. However, these experiments have not been widely used. 

The rf pulses for 27A1 NMR experiments were calibrated using an aqueous solution of A1C13. For the rf power level attenuated by 10 dB, the duration of the 180 "-pulse of the broadband probe was 60 ps. All solids imaging experiments were performed with t => 300 ps and the nominal flip angle a = 90°/(J + 1/2). The two pulses had the same amplitude and for 27A1 MRI were 10- and 20-ps long, respectively. 

Figure 35 Two-dimensional HMBC spectra of isopropylidene glycerol recorded at 400 MHz with (A) the standard HMBC experiment, and (B) with the IMPACT-HMBC experiment. The spectra have been recorded using 128 increments, 2 transients, and a recovery delay of 0.2 s. The flip angle a was set to 130°. Measurement duration for both experiments approximately 2 min.

Fig. 3. HNCA (a) and two implementations of HNCA-TROSY (b-c) experiments for recording intraresidual HN(/), 15N(/), 13C"(i) and sequential 1 HN(7), l5N(/), 13Ca(i — 1) correlations in 13C/15N/2H labelled proteins. Narrow and wide bars correspond to 90° and 180° flip angles, respectively, applied with phase x unless otherwise indicated. Half-ellipse denotes water selective 90° pulse to obtain water-flip-back.88,89 All 90°...

A radio-frequency (rf) alternating field initiates any NMR experiment. At resonance, the field vector B, rotates with Larmor frequency (v, = v0) perpendicularly to the static field vector B0, as shown in Fig. 2.1. In this situation, the nuclear magnetic moments will precess about both fields B0 and B,. Provided B1 extends along the x axis at a certain instant, the double cone of precession will rotate about the x axis in the yz plane (Fig. 2.1 (a) -> (b)). The flip angle 0 relative to the z axis at a given field strength B, depends on the pulse width t of the rf field, in the range of some ps, so that... 

At 60 °C, 25% of the ester groups undergo one (or an odd number of) n-flip at a frequency higher than 10 kHz, with no more than 25° deviation in the flip angle. The flips are accompanied by rotational readjustments with an amplitude of ca. 20° around the local chain axis. The flips occur between energetically inequivalent sites (thus they are active in dynamic mechanical and dielectric relaxation experiments). 

A new type of REDOR experiment (SEDOR) was proposed,104 in which the echo amplitude is measured as a function of the spin flip angle S instead of the dephasing time as in the original REDOR. This flip angle is stepped from 0° to 180°. It is found that this 6-REDOR (SEDOR) can be used to determine how many S spins couple the observed spin I, offering an alternative way of counting spins. 

The delay time between successive scans in a 2D experiment is, of course, of major importance for the sensitivity that can be obtained per unit time. For the case of a one-dimensional experiment in which a single 90° pulse is applied to a spin system and time averaging is used, we saw in Section 3.11 that the optimum time A between consecutive pulses is 1.27T, or alternatively that the Ernst flip angle can be used with more rapid repetition. For most 2D experiments, A = 1.27Tj is also close to optimum for a 90° pulse. This repetition time... 

With the popularity of the pulse Fourier transform nmr spectrometers, recent CIDNP experiments were largely performed in a FT spectrometer. The technique has been reviewed by Kaptein (80), who has emphasized the important facts initially pointed out by Ernst et al. (55) in connection with using a FT spectrometer for CIDNP experiments. For homonuclear multiplet effects a small flip angle of less than 20° should be used to exhibit the multiplet features. As a 90° pulse turns the magnetization vector from the z axis to the xy plane, it would eliminate the homonuclear multiplet effects but not the net effects. The... 

Fig. 25. Dependence of the resolution of MREV spectra on the flip angle /3. The crystal orientation is the same as in the previous experiment (Fig. 24).

Numerical calculations have been carried out on the effect of the angle on the conversion efficiency. The work of Amoureux and Fernandez (1998) is summarised in Table 2.9. One of the key observations is that the efficiency of the flip angles shows no real dependence on r. However, the conversion is quite an inefficient process so that the overall sensitivity of the experiment is poor relative to one-pulse experiments and much effort has gone into improving the excitation and reconversion efficiency. As I increases, the optimum flip angle for maximum excitation decreases. 

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