The single-pulse experiment

In the single-pulse experiments described up to this point, a 90° pulse is followed by a period during which the free-induction decay is acquired (Figure 6-1 a). Fourier transformation of the time-dependent magnetic information into a frequency dimension provides the familiar spectrum of 8 values, henceforth called a one-dimensional (ID) spectrum. 

The amount of N 0 formed is much higher than the amount formed in the single pulse experiments over an oxidised surface. At 573 and 673 K about 50 % of the outcoming NO is detected as N 0. At 513 K 30 % of the outcoming NO is detected as N 0. As the pump/probe pulses are cycled for signal averaging the surface will be partly reduced in this experiment. This result clearly demonstrates that on a partly reduced platinum surface NO will... 

Because has a low natural abundance, NMR measurements on this nucleus require si al averaging. The spin lattice relaxation time, Ti, determines how rapidly a single-pulse I R experiment can be repeated. The rule of thumb is to repeat the experiment every 3Ti to 57 i so that >95% of the equilibrium signal intensity is established between pulses. If we suppose that the T of a C nucleus is 2 min, the single-pulse experiment would be repeated every 6 to 10 min. If 1000 signals must be accumulated to obtain a reasonable signal-to-noise (S/N) ratio, the total experiment time would be between 100 and 167 h. Fortunately, cross-polarization overcomes the problems of the long T s and consequent limited sensitivity. 

Fig. 3 NMR spectra of 5CB at 298 K showing the signal enhancement from the use of adiabatic cross-polarization (b), in comparison to the single-pulse experiment (a). (Reproduced with permission from J. Chem. Phys., 2008, 128, 114504.)...

In the single-pulse experiment, an important parameter is the rate at which the experiment can be repeated, determined by the time it takes to repolarize the system after the magnetization has been destroyed in the excitation and acquisition. This is a... 

Detection of the NMR spectrum of an organic compound in solution with the single pulse experiment, as described above, is quite feasible. However, were the spectrum acquired in this manner, one would notice that the number of peaks in it exceeds the number of unique carbon environments. Most organics contain protons as well as carbon atoms. These protons are coupled to the carbons through the indirect (or scalar) J coupling, resulting in splitting of the carbon resonances of any carbon with attached protons. 

In the single-pulse experiment, the spin system is excited by applying a magnetic field (By) perpendicular to the z axis. The By field is generated by a current in a coil, oriented along the x axis. When the current (rf pulse) is turned on, the magnetisation starts to rotate clockwise around the x axis. 

However, the relaxation contributions obtained from Eq. 22 were not satisfactorily compared with those obtained from specific, deuterium-substitution experiments and single- and double-selective relaxation-rates. Moreover, the errors estimated for the triple-pulse experiments were very much larger than those observed for the other techniques. This point will be discussed next. 

Since the integration values form such an important element of structure determination, we need to set the spectrometer up properly before carrying out the NMR experiment. And one very important parameter which is often forgotten is the relaxation delay, the delay between the single NMR experiments which allows the nuclei to relax. Remember that relaxation is an exponential process, so that theory suggests that it is necessary for the best results to set this equal to at least five times (in our case more than 25 sec for the aromatic protons ). The other parameter we need to set correctly is of course the pulse angle, and the following set of experiments show how these are interrelated. 

The basic experimental arrangement is shown in Figure 1. A Q-switched ruby pump laser is frequency doubled to pump a broadband dye laser with a piano-spherical cavity. The remaining ruby power and the dye pulse were then used for the non-linear spectroscopy experiments. Spectra were recorded on film or plates by means of a Spex Model 1701 spectrograph equipped with a camera. The smoothness of the dye output intensity with wavelength, which determines the sensitivity of the single-pulse spectra, varied from shot to shot and depended on the dye. 

For spin-1 nuclei, both QCPMG and single-pulse MAS experiments were suitable for dynamics studies of nuclei having CqS typical for 2H or 6Li. In case of 14N, the single-pulse MAS experiment was the method of choice for investigation of dynamics for CqS larger than 750 kHz at 14.1 T. 

Two experiments were explored the single-pulse MAS and the QCPMG experiment. The latter may be presented as... 

Figure 12 Maximum intensity as a function of log(fc) for simulated 39K spectra shown in Figures 10 and 11. (A) The intensity profiles for a two-site jump using the QE (solid triangles), the QCPMG (solid line), the QCPMG-MAS (solid circles) and the single-pulse MAS (solid squares) experiments. (B) The intensity profiles of the QCPMG experiment for a two-site (solid line), three-site (solid triangles), four-site (solid squares) and a six-site (solid circles) jump process.

Single pulse experiments, as shown in Fig. 6.2-27, may be performed at different stages of conversion during a polymerization reaction. Analysis of the measured U versus t curves provides a very detailed picture of the polymerization kinetics, e.g., of rate coefficients as a function of monomer conversion (Buback, 1991). 

Henrici et al. [504] carried out a shock tube study of the CO + F2O reaction in mixtures heavily diluted with argon at higher temperatures. They obtained data on overall CO2, O2 and COF2 production from single pulse experiments, and they also made time-resolved optical measurements of the rate of formation of CO2 and depletion of F2O by studying the emission at 4.3 pm and the absorption at 2200 A, respectively. The major path for the decomposition of F2O was assumed to be by reactions (xcii)—(xciv)... 

Reported in Fig. 6 is the normalized fomiation of maleic anhydride (in the sub-second range) in single pulse experiments with an n-pentaney02 f ed on a dean PVO surface (a) and on a PVO... 

Different approaches were used to distinguish between the signals of guest and host molecules. In single-pulse experiments (SPEs), with a relatively short delay of 8-10 s, only solvent C resonances were monitored (Fig. 20a, and top trace Fig. 20c,). This so-called T discrimination experiment takes advantage of the significant distinction of the spin-lattice relaxation times of the mobile... 

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