Line shape distortions

Because polymers are typically non-conductive, sample charging can occur and has to be compensated carefully, e. g. by use of a low-energy electron-flood gun, to avoid line-shape distortion and misinterpretation of the measurements. 

There are several parameters which need to be optimised for the achievement of fhe highest possible CT enhancement with minimal line-shape distortion. With optimal settings, HS and other adiabatic pulses usually give the highest enhancements (close to the theoretical limits of 21 for perfecf ST inversion), when compared to other methods such as DFS and Even without detailed optimisation, it is usually found... 

Finally, one uses low-temperature techniques for ESR measurements of non-Kekule species not only because of the inherent high reactivity of the species, but also because unless the sample is immobilized or at least hampered in its motion by viscous solvent, rapid tumbling will cause the spectrum to suffer line shape distortions that impede the extraction of the zero-field splitting parameters. 

RF pulses and timings were identical to those used in the spectral simulation. Overall, Figure 11 demonstrated good agreement between simulations and experimental data. The small deviations between the simulation and the phantom data were likely caused by T2 differences between different GABA proton groups and line shape distortions from eddy currents (not accounted for in the simulations). The calculated increase in GABA intensity (18%) at 3.01 ppm in the difference spectrum for PRESS+4 compared well with observed a 17% increase in phantom spectrum. 

ESR spectra at X-band were measured with a RE-1006 spectrometer operating at 9,6 GHz (empty cavity at ambient temperature) and 50-lcHz magnetic field modulation. Spectra were recorded at a microwave power of 1 mW and a modulation amplitude 0.02 mT to avoid line shape distortions, which could arise from experimental conditions such as microwave saturation and overmodulation. Scan range was 150-420 mT, Calibration of g-values is based on diphenylpicrylhydracyl (DPPH, g=2.0036) and Cr (g= 1.9796) standards. The amounts of the paramagnetic species (PMS) were calculated by double integration of the resonance line areas. 

As pointed out by Davies and co-workers [91], many high-resolution 2D spectra are not sparse and suffer rather from signal overlap or line shape distortions occurring for several reasons (twisted-shape, truncation artifacts, inhomogeneous broadening). Although these difficulties seem quite different from those in radio astronomy, it became possible to employ essentially the same algorithm to alleviate these problems. 

In laboratory prepared samples, if air is not excluded during the formation of the hydrate, then O2 and N2 can end up in the cages not occupied by the main guest. This shows up as extra electron density in x-ray diffraction single-crystal structural studies, and paramagnetic O2 molecules cause relaxation effects and line-shape distortions in NMR spectra. 

SimFonia Spectra of radicals in fluid solution are calculated using up to third order perturbation theory for isotropic hyperfine couplings. Parameters for the simulation are given in an easy-to-use graphical interface. Instrumental effects such as modulation amplitude and receiver time constant can be included to account for line shape distortions or line broadening. Spectra from SimFonia can be transferred to WinEPR software for editing and output. The software is a commercial product but one version is available free of charge at http //www.bruker-biospin.de/EiPR/software/emx.html. 

In another work the MAS NMR technique is compared to the static powder quadrupole echo (QE) and Jeener-Brockaert (JB) pulse sequences for a quantitative investigation of molecular dynamics in solids. The line width of individual spinning sidebands of the ID MAS spectra were found to be characteristic of the correlation time from 10 to s so that the dynamic range is increased by approximately three orders of magnitude when compared to the QE experiment. As a consequence, MAS NMR is found to be more sensitive to the presence of an inhomogeneous distribution of correlation times than the QE and JB experiments which rely upon line shape distortions due to anisotropic T2 and Tiq relaxation, respectively. All these results have been demonstrated experimentally and numerically using the two-site flip motion of dimethyl sulfone and of the nitrobenzene guest in the a-p-tert-butylcalix[4]arene-nitrobenzene inclusion compound. 

It is not possible to lay down any universal rules since receptivities vary widely and relaxation times may be very long, especially for Li, or extremely short. Where the relaxation times are not too long, data are easy to obtain in the FT mode and the disadvantages of shorter relaxation times and broad lines can often be overcome to a great extent since short acquisition times and rapid pulse repetition rates are permissible, especially if some line-shape distortion can be tolerated. For maximum use to be made of such techniques access to memory sizes less than IK must be possible. 

EPR measurements were made with a Bruker EMX X-band continuous wave spectrometer equipped with a Bruker ER 4116 DM rectangular cavity operating at 9.658 GHz. Experiments were performed at room temperature (300K) with a hyper frequency power of 1 mW and a modulation amplitude of 0.5 G. The amplitude of the magnetic field modulation and microwave power were adjusted so that no line-shape distortion was observable. The received gain was 63,200 and the sweep time was 42 s. Absolute quantification was obtained by comparison with a TEMPO sample of known concentration after double integration of EPR spectra. 

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