Combination peak experiments

Figure 1. Scheme of the pulse EPR sequences mentioned in this chapter, (a) Two-pulse ESEEM. (b) Three-pulse ESEEM. (c) Four-pulse ESEEM. When times fi and ti are stepped under the constraint of ti= ti= T, combination-peak experiment is performed. Two-dimensional HYSCORE spectroscopy is done using the same sequence, whereby t and are stepped independently. The second and third nil pulse are replaced by high-tuming-angle (HTA) pulses in a matched HYSCORE experiment, (d) SMART-HYSCORE. The first and third pulses are HTA pulses, (e) Davies ENDOR. (f) Mims ENDOR. (g) ELDOR-detected NMR. 

Multiple-quantum correlation spectra provide information about through-bond connectivities as all COSY type experiments do. In addition, direct topology information is also available from the same spectrum through remote and combination peaks [5]. Correlation peaks between spins with small chemical shift difference can be examined, too, since there are no diagonal peaks. In this sense, a correlation of MQ coherences with those... 

In the present experiment, we took the aluminum Ka doublet components to be separated by 0.430 eV and to have a half-width of 0.35 eV each. The components were given a 2 1 intensity ratio and scaled to normalize the area under the combined peak. Figure 2 shows the resulting widened sx(x). 

In the basic two-pulse or primary echo experiment, two pulses separated by a time T are applied. The second pulse is twice as long as the first. At time t after the last pulse a transient response appears from the sample, the so called spin echo. By monitoring the echo amplitude as a function of the time t, a spin echo envelope can be recorded. The hyperfine couplings are obtained either by trial-and-error simulations to reproduce the modulations superimposed on the decaying echo amplitude (the original procedure) or by a Fourier transform to obtain nuclear frequencies in modern instruments as in Fig. 2.20. The frequencies are the same as obtained in ENDOR. Contrary to ENDOR, combination peaks at the sum and difference frequencies may also occur. 

Linearity and range Entire method Combination of experiments described under Recovery and Lowest Detectable Quantity. Same sample solutions are prepared as described there. Calibration curve is produced plotting concentration vs. peak height or peak area. Each point is determined from triplicate application of the same concentration of the analyte(s)... 

ENDOR techniques work rather poorly if the hyperfine interaction and the nuclear Zeeman interaction are of the same order of magnitude. In this situation, electron and nuclear spin states are mixed and formally forbidden transitions, in which both the electron and nuclear spin flip, become partially allowed. Oscillations with the frequency of nuclear transitions then show up in simple electron spin echo experiments. Although such electron spin echo envelope modulation (ESEEM) experiments are not strictly double-resonance techniques, they are treated in this chapter (Section 5) because of their close relation and complementarity to ENDOR. The ESEEM experiments allow for extensive manipulations of the nuclear spins and thus for a more detailed separation of interactions. From the multitude of such experiments, we select here combination-peak ESEEM and hyperfine sublevel correlation spectroscopy (HYSCORE), which can separate the anisotropic dipole-dipole part of the hyperfine coupling from the isotropic Fermi contact interaction. 

Three different ID ESEEM schemes using the pulse sequence in Figure 5c and die nuclear CTE have been proposed deadtime-free ESEEM by nuclear coherence transfer echoes (DEFENCE) [24], the combination peak (CP) experiment, and the... 

Figure 6. Examples of ID four-pulse ESEEM experiments, (a) Con arison of three-pulse ESEEM (i) and DEFENCE (ii) experiments of bis(r -benzene)vanadium(0), V(C6H6)2, diluted into polycrystalline ferrocene observer position, gi. (b) Combination-peak spectra of [Cu(H20)6] centers in a frozen water/glycerol solution measured at three different observer positions. The dashed lines mark the frequency 2 Hi. Modified with permission from [24] and [26]. Copyright 1995, American Institute of Physics.

At the time the experiments were perfomied (1984), this discrepancy between theory and experiment was attributed to quantum mechanical resonances drat led to enhanced reaction probability in the FlF(u = 3) chaimel for high impact parameter collisions. Flowever, since 1984, several new potential energy surfaces using a combination of ab initio calculations and empirical corrections were developed in which the bend potential near the barrier was found to be very flat or even non-collinear [49, M], in contrast to the Muckennan V surface. In 1988, Sato [ ] showed that classical trajectory calculations on a surface with a bent transition-state geometry produced angular distributions in which the FIF(u = 3) product was peaked at 0 = 0°, while the FIF(u = 2) product was predominantly scattered into the backward hemisphere (0 > 90°), thereby qualitatively reproducing the most important features in figure A3.7.5. 

We used modifications of the standard solid-state CP-MAS (cross-polarisation, magic-angle spinning) experiment to allow the proton relaxation characteristics to be measured for each peak in the C spectrum. It is known that highly mobile, hydrated polymers can not be seen using either usual CP-MAS C spectrum or solution NMR (6). We found, however, that by a combination of a long-contact experiment and a delayed-contact experiment we could reconstruct a C spectrum of the cell-wall components that are normally too mobile to be visible. With these techniques we were able to determine the mobility of pectins and their approximate spatial location in comparison to cellulose. 

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