NMR pulse sequence

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. 

In the following sections, we will discuss several examples of the 2D NMR pulse sequences to illustrate the essential aspects required to obtain the correlation functions of relaxation and diffusion. 

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],... 

Key Words Binding affinity, H-bonding, Preferred sites, NMR pulse sequence... 

Several NMR pulse sequences have been developed for measuring the diffusion coefficient. The most successful and currently used experiment is the modified spin-echo experiment (PGSE) proposed by Stejskal in 1967.62 As the echo signal is due to the transversal components of magnetic moments, the echo attenuation normalized for the attenuation due to relaxation can be written as63 ... 

Within various pharmaceutical laboratories (industrial and academic), the mul-tinuclear technique of solid state NMR has primarily been applied to the study of polymorphism at the qualitative and quantitative levels. Although the technique ideally lends itself to the structure determination of drug compounds in the solid state, it is anticipated that in the future, solid state NMR will become routinely used for method development and problem solving activities in the analytical/materials science/physical pharmacy area of the pharmaceutical sciences. During the past few years, an increasing number of publications have emerged in which solid state NMR has become an invaluable technique. With the continuing development of solid state NMR pulse sequences and hardware improvements (increased sensitivity), solid state NMR will provide a wealth of information for the physical characterization of pharmaceutical solids. 

Figure 1. Schematic diagram of the solid-state NMR pulse sequences for (a) quantitative single pulse 13C observe with gated decoupling and (b) Ti and (c) 13C Ti determinations via cross polarization.

Scalar coupled experiments COSY and TOCSY The correlated spectroscopy (COSY) experiment is one of the most simple 2D-NMR pulse sequences in terms of the number of RF pulses it requires [32]. The basic sequence consists of a 90-C-90-acquire. The sequence starts with an excitation pulse followed by an evolution period and then an additional 90° pulse prior to acquisition. Once the time domain data are Fourier transformed, the data appear as a diagonal in... 

Until now the mentioned approaches needed the NMR pulse sequence to be triggered with respect to the mechanical wave in order to avoid inappropriate wave detection due to the value of cp. Recently a method authorizing a proper detection of transverse waves without synchronization was... 

It is also possible to screen catalysts according to their selectivities (regio- and stereoselectivity), their stabilities, and rates, since kinetic investigations can be carried out as well [23]. In addition, the application of especially designed NMR pulse sequences further extends the possibilities of the PHIP method [19]. For all of these applications, the main advantage of PHIP is the remarkable signal enhancement which is combined with a spectroscopic in situ method of observation. 

Figure 6.80 A basic NMR pulse sequence. Reprinted, by permission, from C. Westbrook and C. Kaur, MRI in Practice, 2nd ed., p. 15. Copyright 1998 by BlackweU Science Ltd.

The rates of acid-catalysed medioxy exchange between methanol and the three diastereomers of 2-medioxy-4,6-dimethyl-l,3-dioxalane (37a-c) were measured in benzene and methanol-chloroform. Rate constants were evaluated in a novel way using 1D-EXSY NMR pulse sequence and a weighted least-squares analysis. The critical intermediate is (38) and rates of methanol attack on it in benzene show a 24-fold axial selectivity whereas in methanol-chloroform the selectivity difference is ninefold.34... 

These types of experiments call for efficient doubly tuned coils, ideally with a separate deuterium lock channel. For more complex molecules, such as proteins, considerably more intricate NMR pulse sequences, such as (HNCO) [32,33], require the probe to operate at three or four distinct frequencies. High efficiency is demanded from the proton observe channel. Ideally, the additional circuitry allowing multiple tuning should not interfere with the proton efficiency when compared to a singly tuned proton coil. In practice, some reduction is tolerated. The two most important design criteria for such... 

Various NMR pulse sequences contain certain building blocks like BIRD, INEPT, refocusing delay etc., that perform some specific functions. A building block that serves... 

CH2, or CH3) is achieved with a ubiquitous building block of NMR pulse sequences the spin echo. 

NMR pulse sequence without getting tied up in the details of pulse phases and a mountain of sine and cosine terms only the essential elements of the sample net magnetization will be described at each point. Finally, the formal Hamiltonian description of solution-state NMR will be described and applied to explain two related phenomena strong coupling ( leaning of multiplets) and TOCSY mixing (the isotropic mixing sequence). 

Now that we know most of the basic building blocks of NMR pulse sequences, we should be able to use the coherence flow diagram (Fig. 11.9) to design an HSQC pulse sequence. It needs to accomplish the following steps ... 

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