Pulse, read

The first term represents inverted z magnetization, the starting point for a transient NOE experiment. All we have to do is add a mixing period (simple delay rm) and a 90° read pulse, and we have a 2D NOE experiment NOESY. For the COSY experiment, we start... 

Figure 10 Wet pulse sequences. Cartoon representations of the (A) basic WET, (B) WET with composite (90 90 90 90 ) read pulse, (C) WET180, (D) WET270, and (E) SWET pulse sequences. See Figure 6 for pulse descriptions and text for details. WET gradients were the same for all sequences and are described in text.

The inverse INEPT (Bodenhausen and Ruben, 1980) and inverse DEPT (Brooks et ai, 1984) experiments utilize such an approach. In the inverse INEPT experiment, successive 90° pulses are applied to the C nucleus, followed by a H read pulse. Protons not coupled to the C nucleus are suppressed by presaturation of the entire H-NMR spectrum before the polarization transfer, so only those signals will be detected that are generated by polarization transfer from the C nucleus. 

Figure 3 Creation of the longitudinal order by cross-correlation as a function of the mixing time fm which follows the inversion of a carbon-13 doublet (due to a./-coupling with a bonded proton). The read-pulse transforms the longitudinal polarization into an in-phase doublet and the longitudinal order into an antiphase doublet. The superposition of these two doublets leads to the observation of an asymmetric doublet.

Finally, the double quantum (DQ) spin spin relaxation time T2d can be determined using the pulse sequence 90° — x — 45° — 90° —t — 45°.51 The first three pulses create the DQ coherence, and the last read pulse converts the DQ to a single quantum coherence for detection. 

Fig. 1. Top Scheme of an inversion recovery experiment 5rielding the longitudinal relaxation time (inversion is achieved by mean of the (re) radiofrequency (rf) pulse, schematized by a filled vertical rectangle). Free induction decays (fid represented by a damped sine function) resulting from the (x/2) read pulse are subjected to a Fourier transform and lead to a series of spectra corresponding to the different t values (evolution period). Spectra are generally displayed with a shift between two consecutive values of t. The analysis of the amplitude evaluation of each peak from — Mq to Mq provides an accurate evaluation of T. Bottom the example concerns carbon-13 Tl of irans-crotonaldehyde with the following values (from left to right) 20.5 s, 19.8 s, 23.3 s, and 19.3 s.

This latter form is especially convenient for a quick determination of Ti, which is the inverse of the slope of the linear representation of In vs. X. Mq can be measured by means of a single read-pulse or for... 

As stated before, the initial perturbation should be maximal with respect to the equilibrium state. Since we are dealing with transverse magnetization here, this maximal perturbation is obviously a 90° pulse. However, it can be immediately noticed that signals collected after a simple read-pulse, decay exponentially according to a time constant which differs from the genuine T2 by a contribution due to the static induction Bq inhomogeneity ... 

The experiment is very simple—one line of a multiplet is selectively inverted (by a selective 180° pulse of duration r) immediately before the usual non-selective read pulse is applied and the FID sampled. The Fourier transform then yields the whole spectrum with altered line intensities. Populations of the energy levels connected by the selectively irradiated line are changed (inverted) by the selective pulse, and this leads to changes in... 

The final read pulse rotates the Hb z magnetization into the x -y plane and the FID is recorded with the frequency Fourier transformation of the FIDs gives in each one a peak at F2 = 2b whose amplitude is oscillating as a function of t at the frequency 2a. Fourier transformation of the t FID gives a crosspeak at F2 = 2b, F = 2a. Because for small molecules, the transferred z magnetization is opposite in sign from the original... 

The DANTE pulse sequence is shown in Figure 20b. A peak that is exactly on-resonance or an integral multiple of l/d2 Hz off-resonance, experiences the cumulative effect of the n pi pulses. Other resonances see a random excitation that effectively sums to zero. In exchange experiments, a variable delay (vd) is inserted between the end of the DANTE pulse train and the read pulse, p2. During vd, magnetization is transferred between exchanging sites. 

A Varian Unity Inova 600-MHz NMR instrument (Palo Alto, CA) equipped with a H C/ N pulse field gradient triple resonance microliow NMR probe (flow cell 60pL 3mm O.D.) was used. Reversed-phase HPLC of the samples was carried out on a Varian modular HPLC system (a 9012 pump and a 9065 photodiode array UV detector). The Varian HPLC software was also equipped with the capability for programmable stop-flow experiments based on UV peak detection. An LCQ classic MS instrument, mentioned in the previous section, was connected on-line to the HPLC-UV system of the LC-NMR by contact closure. The H resonance of the D2O was used for field-frequency lock, and the spectra were centered on the ACN methyl resonance. Suppression of resonances from HOD and methyl of ACN and its two C satellites was accomplished using a train of four selective WET pulses, each followed by a Bo gradient pulse and a composite 90-degree read pulse [41]. 

We can also show that a similar procedure can be used in sequences where spin dephasing is carried out using the gradient of rf field, B,(r, t), instead of using a magnetic field gradient (Canet et al., 1989). For example, the sequence of two rf gradient pulses with an intermediate tt pulse and v/2 read pulse can be described by... 

In the context of electron-transfer reactions, the contribution to the body of knowledge on the kinetics and mechanisms of these processes from pulse radiolysis studies is very significant. As has been shown above and elsewhere (see the list of Further Reading ), pulse radiolysis is particularly well suited to the study of fast electron-transfer reactions involving free radicals and metal ions in unusual oxidation states. 

Flip angles are determined by the specific 2D experiment being conducted, and Erast angle considerations are not an issue. The COSY family of experiments has a variety of final pulses (Section 6-1), and they are considered in Section 7-7a. Moreover, the final pulse before that is delivered to the detected nucleus is known as the read pulse,... 

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