Pulse sequence experiment

Highly sophisticated pulse sequences have been developed for the extraction of the desired information from ID and multidimensional NMR spectra [172]. The same techniques can be used for high-resolution 1-NMR, s-NMR and NQR. Pulse experiments are commonly used for the measurement of relaxation times [173], for the study of diffusion processes [174] and for the investigation of chemical reactions [175]. Davies et al. [176] have described naming and proposed reporting of common NMR pulse sequences (IUPAC task group). An overview of pulse sequence experiments has been given [177],... 

Although the SPI experiment is not too useful by itself, we will discover that the technique of polarization by population inversion is central to many of the more advanced pulse sequence experiments. 

NMR experiments are an ideal tool for studying the structure and dynamics of liquids, because the measurement itself does not perturb the molecular properties of the system. Furthermore, by selectively studying the nuclear spin of different elements, one can probe different part of the system. In combination with advanced pulse sequence experiments different interactions and processes can be studied. 

Fig. 8. NMRof PinP-dopeda-Si Hwith 1.8 mol %P. The top trace is from the FID and the middle trace from an echo (90-t- 180 pulse sequence) experiment. The bottom trace represents the difference between the top two traces as described in the text. [From Reimer and Duncan (1983).]...

The single most important development in nuclear magnetic resonance (NMR) spectroscopy since the initial observation of the NMR phenomenon in bulk phases in 1945 was undoubtedly the introduction of pulse Fourier transform NMR by Anderson and Ernst. This technique provided greatly increased sensitivity per unit time, making it feasible to obtain spectra for low sensitivity/low abundance nuclei such as More importantly, it allowed the development of a wide variety of sophisticated and powerful multipulse experiments which have revolutionized the use of NMR spectroscopy in studies of molecular structure and dynamics. This article provides an overview of pulse sequence experiments. Many individual experiments are discussed in other articles. 

Mastery of this approach is essential for anyone desiring to design new pulse sequence experiments and valuable for anyone wishing to understand modern NMR experiments. 

Figure Al.6.22 (a) Sequence of pulses in the canonical echo experiment, (b) Polarization versus time for the pulse sequence in (a), showing an echo at a time delay equal to the delay between the excitation pulses.

Figure Bl.12.12. Pulse sequences used in multiple quantum MAS experiments and their coherence pathways for (a) two-pulse, (b) z-filter, (c) split-t with z-filter and (d) RIACT (II).

Advantages. The experiment can be readily carried out with a conventional probe-head, although the fastest spiiming and highest RF powers available are usefid. The pulse sequences are relatively easy to set up (compared to DAS and DOR) and the results are usually quite straightforward to interpret in temis of the number of sites and detemiination of the interactions. 

A measure of the echo attenuation within each pixel of an image created using the pulse sequence of figure Bl.14,9 perhaps by repeating the experiment with different values of and/or 8, gives data from which a true diffusion map can be constructed [37, 38],... 

Figure Bl.15.11. Fomiation of electron spin echoes. (A) Magnetization of spin packets i,j, /rand / during a two-pulse experiment (rotating frame representation). (B) The pulse sequence used to produce a stimulated echo. In addition to this echo, which appears at r after the third pulse, all possible pairs of the tluee pulses produce primary echoes. These occur at times 2x, 2(x+T) and (x+2T).

In electron spin echo relaxation studies, the two-pulse echo amplitude, as a fiinction of tire pulse separation time T, gives a measure of the phase memory relaxation time from which can be extracted if Jj-effects are taken into consideration. Problems may arise from spectral diflfrision due to incomplete excitation of the EPR spectrum. In this case some of the transverse magnetization may leak into adjacent parts of the spectrum that have not been excited by the MW pulses. Spectral diflfrision effects can be suppressed by using the Carr-Purcell-Meiboom-Gill pulse sequence, which is also well known in NMR. The experiment involves using a sequence of n-pulses separated by 2r and can be denoted as [7i/2-(x-7i-T-echo) J. A series of echoes separated by lx is generated and the decay in their amplitudes is characterized by Ty. ... 

More sophisticated pulse sequences have been developed to detect nuclear modulation effects. With a five-pulse sequence it is theoretically possible to obtain modulation amplitudes up to eight times greater than in a tlnee-pulse experunent, while at the same time the umnodulated component of the echo is kept close to zero. A four-pulse ESEEM experiment has been devised to greatly improve the resolution of sum-peak spectra. 

An alternative 2D ESEEM experiment based on the four-pulse sequence depicted in figure B 1.15.12(B) has been proposed by Meliring and coworkers [40]. In the hyperfme sublevel correlation (HYSCORE) experiment, the decay of the echo intensity as a fimction of is governed by -relaxation, whereas the echo decay along the t2-axis is... 

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... 

The ID homonuclear Hartmann-Hahn (HOHAHA) experiment is an excellent way to determine complete coupled spin networks (18). The following pulse sequence is used ... 

The pulse sequence which is used to record CH COSY Involves the H- C polarisation transfer which is the basis of the DEPT sequence and which Increases the sensitivity by a factor of up to four. Consequently, a CH COSY experiment does not require any more sample than a H broadband decoupled C NMR spectrum. The result is a two-dimensional CH correlation, in which the C shift is mapped on to the abscissa and the H shift is mapped on to the ordinate (or vice versa). The C and //shifts of the //and C nuclei which are bonded to one another are read as coordinates of the cross signal as shown in the CH COSY stacked plot (Fig. 2.14b) and the associated contour plots of the a-plnene (Fig. 2.14a and c). To evaluate them, one need only read off the coordinates of the correlation signals. In Fig. 2.14c, for example, the protons with shifts Sh= 1.16 (proton A) and 2.34 (proton B of an AB system) are bonded to the C atom at c = 31.5. Formula 1 shows all of the C//connectivities (C//bonds) of a-pinene which can be read from Fig. 2.14. 

HC HMQC (heteronuclear multiple quantum coherence) and HC HSQC (heteronuclear single quantum coherence) are the acronyms of the pulse sequences used for inverse carbon-proton shift correlations. These sensitive inverse experiments detect one-bond carbon-proton connectivities within some minutes instead of some hours as required for CH COSY as demonstrated by an HC HSQC experiment with a-pinene in Fig. 2.15. 

The SP-PLP817 18 and PS-PLP171 techniques involve following the monomer conversion induced by a single laser pulse or a sequence of laser pulses. These experiments are usually conducted at high pressure beeause rates of termination are lower and sensitivities are somewhat higher. 7... 

Fig. 2. The pulse sequence for the CP/MAS experiment. The values of the different time parameters depend on the relaxation behaviours and on the mobilities of the nuclei in the compounds investigated. (Reproduced with permission of Ref. I0))...

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