Second pulse sequence

Figure 22. Magnitude of the excited-state wavefunction at t = 800 a.u., for the second pulse sequence described in the text (pulse delay = 825 a.u., A = B = 0.125). The excited state wavefunction at t = 200, 400, and 600 a.u. is virtually identical to that in Figs. 20a-20c.

The second pulse sequence (HCCH) provides detection of 2D-INADEQUATE spectra in the /7/2 planes wiA detection of H in If a particular H chemical shift is selected, the flJ2 plane will contain the INADEQUATE spectrum of the directly boimd and adjoining. atoms. Because the experiments were performed on labeled polymers, the sensitivity was irly high, however, this experiment does not filter resonances of the polymer backbone from the spectrum. Practical aspects of using these pulse sequences are described by Saito et al.(20)... 

The pulse sequence (stages 2-3) can be repeated hundreds of times to enhance the signal to noise ratio The duration of time for stage 2 is on the order of milliseconds and that for stage 3 is about 1 second... 

The key dimension m NMR is the frequency axis All of the spectra we have seen so far are ID spectra because they have only one frequency axis In 2D NMR a stan dard pulse sequence adds a second frequency axis Only pulsed FT NMR spectrometers are capable of carrying out 2D experiments... 

Fig. 6. The generalized Jeener-Broekaert three pulse sequence. Note that FT of the solid echo and the alignment echo starts at times delayed by the pulse separation r, after the second and third pulse, respectively...

The first step in determination of a structure by NMR spectroscopy involves assignment of individual proton resonances. Development of high-field spectrometers and the use of a second dimension (2D-NMR) along with isotopic substitution (11) and sophisticated pulse sequences (12) make it possible to almost completely assign the proton spectrum of proteins of about 15 kD molecular weight (13—17). Some 2D-pulse sequences commonly used in the study of macromolecules are shown in Figure 1. 

To determine molecular motions in real time necessitates the application of a time-ordered sequence of (at least) two ultrafast laser pulses to a molecular sample the first pulse provides the starting trigger to initiate a particular process, the break-up of a molecule, for example whilst the second pulse, time-delayed with respect to the first, probes the molecular evolution as a function of time. For isolated molecules in the gas phase, this approach was pioneered by the 1999 Nobel Laureate, A. H. Zewail of the California Institute of Technology. The nature of what is involved is most readily appreciated through an application, illustrated here for the photofragmentation of iodine bromide (IBr). 

In the one-dimensional NMR experiments discussed earlier, the FID was recorded immediately after the pulse, and the only time domain involved (ij) was the one in which the FID was obtained. If, however, the signal is not recorded immediately after the pulse but a certain time interval (time interval (the evolution period) the nuclei can be made to interact with each other in various ways, depending on the pulse sequences applied. Introduction of this second dimension in NMR spectroscopy, triggered byjeener s original experiment, has resulted in tremendous advances in NMR spectroscopy and in the development of a multitude of powerful NMR techniques for structure elucidation of complex organic molecules. 

Jeener s idea was to introduce an incremented time ti into the basic ID NMR pulse sequence and to record a series of experiments at different values of second dimension to NMR spectroscopy. Jeener described a novel experiment in which a coupled spin system is excited by a sequence of two pulses separated by a variable time interval <]. During these variable intervals, the spin system is allowed to evolve to different extents. This variable time is therefore termed the evolution time. The insertion of a variable time period between two pulses represents the prime feature distinguishing 2D NMR experiments from ID NMR experiments. 

Many variations of this experiment are known. Some of the pulse sequences used for recording heteronuclear 2D/resolved spectra are shown in Fig. 5.8. In a modified gated decoupler sequence (Fig. 5.8b), the decoupler is off during the first half of the evolution period and is svdtched on during the second half. Any C resonances that are folded over in the F, domain may be removed by employing the fold-over corrected gated decoupler sequence (FOCSY) (Fig. 5.8c) or the refocused fold-over corrected decoupler sequence (RE-FOCSY) (Fig. 5.8d). 

The delay is generally kept at Vi x> The coupling constant Jcc for direcdy attached carbons is usually between 30 and 70 Hz. The first two pulses and delays (90J -t-180 2-t) create a spin echo, which is subjected to a second 90J pulse (i.e., the second pulse in the pulse sequence), which then creates a double-quantum coherence for all directly attached C nuclei. Following this is an incremented evolution period tu during which the double quantum-coherence evolves. The double-quantum coherence is then converted to detectable magnetization by a third pulse 0,, 2, and the resulting FID is collected. The most efficient conversion of double-quantum coherence can... 

Figure 7.17 Pulse sequence for soft H,H-COSY with two selective pulses exciting the / multiplet and the second mixing pulse exciting the li multiplet of a three-spin system (/, /,)). (Reprinted from Mag. Reson. Chem. 29, H. Kessler et al., 527,...

The sensitivity and specificity of DWI depend to some extent on the technique being used and the amount of imaging time that can be dedicated to the DWI sequence. DWI pulse sequences typically require between approximately 30 seconds and 4 minutes of imaging time to image the entire brain and achieve sensitivity and specificity approaching 100% (Fig. 2.2). The rare infarcts that are not apparent on DWI are usually very small and are often located in the brainstem. 

Fig. 2.6.7 General principle of time-of-flight flow detection, (a) Schematic of a set-up for TOF experiments. An object of interest is placed inside an environment optimized for encoding (field gradients not shown). As the sensor medium flows out of the analyte object, its magnetization is recorded with a second coil with a smaller volume, which is placed as close to the encoding volume as possible, (b) Generic pulse sequence used for TOF experiments. Encoding along one dimension can be done by inverting the magnetization of a slice...

Fig. 5.3.4 (A) Stimulated echo dynamic NMR microscopy pulse sequence. The first field gradient pulse (g,) of duration 8 serves to encode spatial positions of spins and the second field gradient pulse has a refocusing effect.

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