Pulse sequence 5" -order

The practical goal for pulsed EPR is to devise and apply pulse sequences in order to isolate pieces of infomiation about a spin system and to measure that infomiation as precisely as possible. To achieve tliis goal it is necessary to understand how the basic instnunentation works and what happens to the spins during the measurement. 

B2.5.351 after multiphoton excitation via the CF stretching vibration at 1070 cm. More than 17 photons are needed to break the C-I bond, a typical value in IR laser chemistry. Contributions from direct absorption (i) are insignificant, so that the process almost exclusively follows the quasi-resonant mechanism (iii), which can be treated by generalized first-order kinetics. As an example, figure B2.5.15 illustrates the fonnation of I atoms (upper trace) during excitation with the pulse sequence of a mode-coupled CO2 laser (lower trace). In addition to the mtensity, /, the fluence, F, of radiation is a very important parameter in IR laser chemistry (and more generally in nuiltiphoton excitation) ... 

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... 

This phase shift depends on x0, vx0 and ax0 at the same time, i.e., on the parameters needed for spin density, velocity and acceleration mapping, respectively. In order to distinguish the three quantities of interest, more sophisticated gradient pulse sequences have been designed, resulting in phase shifts that essentially depend only on a single parameter or a selection of parameters. 

The flow artifacts detected in the droplet size measurements are similar to those reported by Goux et al. [79] and Mohoric and Stepisnik [80]. In their work natural convection effects led to an increase in the decay of signal attenuation curves, causing over-prediction in the self-diffusion coefficient of pure liquids. In order to avoid flow effects in droplet size distributions, flow compensating pulse sequences such as the double PGSTE should be used. It has been demonstrated recently that this sequence facilitates droplet size measurements in pipe flows [81]. 

Alternatively, proton double quantum (DQ) NMR, based on a combined DQ excitation and a reconversion block of the pulse sequence, has been utilized to gain direct access to residual DCCs for cross-linked systems.69,83-89 For this purpose, double-quantum buildup curves are obtained with use of a well-defined double-quantum Hamiltonian along with a specific normalization approach. Residual interactions are directly proportional to a dynamic order parameter Sb of the polymer backbone,87... 

Figure 22 Pulse sequence of the HMBC-RELAY experiment. Filled and open bars represent 90° and 180° pulses, respectively. All other phases are set as x, excepted otherwise stated. A two-phase cycle x, —x is used for the pulse phases (j>, and

Figure 27 Edited broadband HMBC spectrum of cyclosporine using the pulse sequences shown in Figure 26 in an interleaved manner. The two subspectra, CH + CH3 (left) and C + CH2 (right), exemplify the editing properties. The spectrum in the bottom displays the two subspectra, CH + CH3 (black) and C + CH2 (grey) in the same frame. The number of scans was 32 for each of the 128fi increments, the relaxation delay was 1 s, and the range for the third-order low-pass. /-filter was 115 Hz < Vch < 165 Hz. The spectra were processed to maintain the absorptive profiles in F, while a magnitude mode was done in F2.

The final 2D spectra obtained by shearing or by using the split-/, pulse sequence can benefit from a proper scaling of the isotropic and anisotropic dimensions in order to facilitate comparisons between various experiments [171, 176-179]. In our opinion, the most convenient way to reference such 2D spectra is to define a ppm scale using, in the isotropic dimension, an apparent Larmor frequency depending on the given experiment and defined as... 

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