Lineshap tests

The proton lineshape test uses chloroform in deuteroacetone typically at concentrations of 3% at or below 400 MHz, and 1% at or above 500 MHz. Older instruments and/or probes of lower sensitivity or observations via outer decoupler coils , may require 10% at 200 MHz and 3% at 500 MHz to prevent noise interfering with measurements close to the baseline. A single scan is collected and the data recorded under conditions of high digital resolution (acquisition time of 16 s ensuring the FID has decayed to zero) and processed without window functions. Don t be tempted to make measurements at the height of the satellites themselves unless these are confirmed by measurement to be 0.55%. Since these arise from protons bound to C, which relax faster than those of the parent line, they may be relatively enhanced should full equilibrium not be established after previous pulses. The test results for a 400 MHz instrument is shown in Fig. 3.66. The traditional test for proton resolution which dates back to the CW era (o-dichlorobenzene in deuteroacetone) is becoming less used nowadays, certainly by instrument manufacturers, and seems destined to pass into NMR history. 

Figure 3.66. Proton resolution and lineshape tests for a static 3% CHCI3 sample recorded on a 400 MHz spectrometer equipped with a dual H C inverse probehead. The C satellites are clearly seen in (b) at 109 Hz, whereas the artefacts at 20 Hz arise from floor vibrations.

In the interpretation of these spectra it is assumed that the geometry (i.e., the layer statistics in the NMR layer model) of the particles does not change as a result of chemisorption but that the surface LDOS and the healing length are affected. In principle, this suggestion can be tested experimentally by comparing the lineshape in a double-resonance experiment on... 

The early speculations about transition moment directions in the lower aromatics needed to be tested experimentally. The traditional vapour phase measurements of band lineshapes were not accessible with the spectroscopic resolving power we then had, but came later (sec 4.2). Most absorption systems of aromatics in the UV were in any case diffuse and structureless. The method that could be used was to get the polarization of the light absorbed by the crystal composed of the target molecules. The positions and orientations of the molecules in the laboratory frame were known from X-ray crystal structure analysis. The interpretation of results raised non-trivial theoretical problems of its own. The experiments will be described first. 

Constant-time imaging of H in solids has been applied in combination with MAS for line narrowing [Cor3, Cor4]. The images were recorded for a dynamically stressed poly(isoprene) phantom and of poly(butadiene) in two poly(butadiene)/poly(styrene) blends. Spectroscopic MAS imaging has also been tested on deuterated polymers to probe differences in molecular moblity from the lineshape of the rotary-echo envelope... 

Lineshape and resolution tests on other nuclei follow a similar procedure to that above. Not all nuclei available with a given probe need be tested and typically only tests for inner and outer coil observations on multinuclear probes are required. This means the second test will often involve carbon-13 for which two samples are in widespread use the ASTM (American Society for Testing and Materials) test sample (40% p-dioxane in deuterobenzene also used for the sensitivity test) or 80% benzene in deuteroacetone. In either case on-resonance continuous-wave (CW) decoupling of protons should be used as this provides improved results for a single resonance relative to broadband decoupling. Rather long (30-40 s) acquisition times will be required for a well shimmed system. 

The experimental test of this model requires a systematic investigation of the dependence core level lineshapes on the above parameters A and A +, and a... 

The essential physical condition for a meaningful test of the simulation is an experimental spectrum that meets the assumptions of the simulation. The NMR lineshape should result from the inhomogeneous distribution of geometries and should not be distorted by experimental effects (motional broadening, insufficient proton decoupling, transversal relaxation, CP efficiency). Complete motional averaging (i.e. liquid-like behavior), however, can easily be taken into account. 

As a test of the lineshapes, we have fitted Eqs. (62-65) to a variety of experimental data, using F as an adjustable parameter and choosing (Oki be the lowest-lying allowed molecular transition frequency. " The resonance frequency is selected so that the peak of the observed profile matches the calculated curve, and the surface concentration of adsorbed species is assumed to be relatively independent of potential in this region. In Figure 20, we show... 

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