Pulse sequence definition

Fig. 1. Two showing the fifth-order Raman pulse sequence. (A) Definition of fields, (B) Energy level diagram showing one possible Liouville space pathway.

Varian uses the decoupler mode (dm) parameter to determine when the decoupler should be turned on and off during the pulse sequence. Time periods A, B, C,. .. are defined during any pulse sequence, and the definition of these time periods can be observed by entering dps (display pulse sequence). For the general ID sequence called s2pul (simple 2-pulse), the periods A, B, and C are defined as follows ... 

The ESE method was used to identify the overlapping ESR spectra of the irradiated PMMA more definitely. The measurements of the ESE-detected ESR were made at 77 K with the 90° — t — 180 two-pulse sequence, at various fixed times of longitudinal relaxation, tl5 while the external magnetic field was swept slowly. The t( -resolved ESR spectrum was obtained from the difference between the echo intensity at a fixed x of 0.5 ps with and without a 90° saturation pulse... 

Fig. 14-9. Definition of parameters and pulse sequences applied in Ref. [21]. Here, the A t) values are averaged during the time window.

In the context of the NMR-SIM simulation, the recording of a one-dimensional experiment consists of the definition of the spin system, the pulse sequence and the experiment parameters. The result of the simulation is a FID that is loaded into the ID WIN-NMR processing program. Both the time domain and the frequency domain data may be processed using a variety of domain specific mathematical functions. 

In the first section of this chapter spin systems and pulse sequences are discussed. Starting with the definition of a spin system and a description of the various spin parameters used by NMR-SIM. Using suitable examples the correct definition of spin system parameters, how to reduce the number of spin system parameters and the application of variable spin system parameters is illustrated. Following on, pulse sequences are discussed in some detail and the difference between a pulse sequence and a pulse program is emphasized. The pulse program language of NMR-SIM should be familiar to users of Bruker NMR spectrometers as it uses the same syntax. However, because this is not covered in the NMR-SIM manual, pulse sequence elements such as pulses, phases and delays are explained in detail and illustrated using a variety of examples. 

The command molecule has the highest priority in the internal hierarchy of the spin system definition. Next comes the nucleus statement, each magnetically non-equivalent nucleus is defined, one nucleus per line followed by the coupling interaction between nuclei, one interaction per line. It is also possible to define a non-thermal equilibrium spin system state such as produced in multiple quantum coherence experiments. The required coherence may be selected using the spin state definition rather than by a pulse sequence, this not only simplifies the pulse sequence but also reduces calculation times. Fig. 4.3 illustrates the general layout of a spin system file. In the case of a single spin system, the molecule and endmol commands are redundant and may be omitted. 

It is also possible to study the effect of pulse sequences using the variable definition feature. Using variable definitions it is possible to increment the chemical shift to... 

The operators and arguments loto and tdl times corresponds to the expressions used in the pulse sequence language described in section 4.1.2.3. Table 4.2 summarizes the parameter definition, initialisation and variable parameter operators. 

Pulses, delays and gradients are the basic elements of a pulse sequence. They are often used repeatedly and may be changed in a very versatile way. Their exact definition and time related execution follows a specific set of rules which the pulse program interpreter uses to translate the line ordered execution commands into a computer readable list of commands. 

Load the configuration file dsweep.cfg. Open the spin system file (Edit Spin system...) and note the spin system variable definition. Examine the pulse sequence (Edit Pulse program...) and note the additional loop command to increment the chemical shift. Using the GolRun Experiment command simulate the spectrum which corresponds to the excitation profile of a 90° TOPHAT pulse. In 1D WIN-NMR process the FID using zero filling of Sl(r+i) 32/cand an exponential window function with a LB value of 2 Hz. 

The first step is to construct a partial pulse program with pulses and delays in a simple sequence. As users of BRUKER spectrometers are already aware, the scheme of the DEPT experiment imposes a number of restrictions on the pulse sequence the rf channel used for detection is designated El while the second rf channel is designated F2. For each pulse in the pulse sequence the pulse length, pulse phase and rf channel has to be specified as for example (pi phl) fl or (p3 ph3) fl. As shown in the table below pulses pi to p4 are generally used to describe 90° and 180° pulses, however it is strongly recommended that these pulse definitions are used in all pulse sequences. This approach not only helps other users understand the pulse program more easily and allows the experiment parameters to be set up faster, but more importantly it allows the use of the NMR-SIM s NMR Wizard. 

The final Check it in this section draws together a number of different threads using spin system definitions and pulse sequences from other Check its to simulate both the gradient selected IR HMQC spectrum and the fl/f2 projections of... 

At the start of this section it was stressed how important it was to add comments both to spin system definitions and pulse sequences. Comments not only improve the "readability of pulse programs but also indicate what modifications have been made to standard sequences as illustrated by the examples in this section. In a complex pulse... 

Because NMR-SIM uses simple text files for both the definition of spin system parameters and for writing pulse sequences, it offers a straightforward method for investigating and comparing filter elements. In addition, the results are presented in a graphical format in the form of NMR spectra, which are easier to interpret than the results obtained from a purely mathematical simulation. 

If the carrier frequency of the RF pulses is close to one of the resonance transitions of the irradiated sample, the effect of the multi-pulse sequence leads to the establishment of a new time independent effective Hamiltonian Heff, now also containing the interaction of nuclear-spins with the RF field (that the effective Hamiltonian is time independent should be imderstood that it is independent of the number of the pulse interval but not independent of the time inside the definite pulse interval). A part of the dipole Hamiltonian Ha no longer commutes with the new effective Hamiltonian, which after the T2 time results in heat mixing of the quadrupole reservoir and the reservoir of the components of... 

Virtually all NMR experiments can described in terms of a pulse sequence, which, as the name suggests, is a notation which describes the series of radiofrequency (rf) or field-gradient pulses used to manipulate nuclear spins and so tailor the experiment to provide the desired information. Over the years, a largely (although not completely) standard pictorial format has evolved for representing these sequences, not unlike the way a musical score is used to encode a symphony. As these crop up repeatedly throughout the text, the format and conventions used in this book deserve explanation. Only the definitions of the various... 

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