NMR resolution

Figure 7.22 shows the H NMR chromatogram (contour plot) of the separation of a 10% phthalate mixture in CH2CI2. The spectrum is almost free from interferences the NMR resolution is excellent, and it is possible to identify all plasticisers even at concentrations as low as 2%, which corresponds to 60 xg per component. In contrast, in on-line HPLC- H NMR separation the regions between 3.9-3.3 and 1.9-1.7 ppm are completely obscured by solvent signals. 

In principle, therefore, the microstructure of such a polymer could be described by a sequence of m and r diads. However, the limits of NMR resolution is such that the microstructure can only be determined over a limited section of the polymer, usually no longer than 5-7 monomer units. NMR spectra on comparatively insoluble polymers such as polyethylene and polypropylene are usually recorded in 1,2,4-trichlorobenzene or tetrachloroethane-d2 at elevated temperatures (110-150 °C). 

Figure 46 (Top) Experimental 31P NMR spectra of three component sample crystallised from toluene (A) powdered sample (PS) recorded with CP/MAS sequence and spinning rate 8 kHz (B) three-component single crystal (TCSC) A held in rotor filled with silica gel recorded with CP/MAS sequence and spinning rate of 8 kHz. Note the much better NMR resolution of resonance lines and small distinction of 31P chemical shifts for the monocrystal compared to powdered sample. (Bottom) 31P-31P proton-driven spin diffusion 2D correlation recorded with mixing times of (A) 0.2 and (B) 10 s. Taken from Ref. [229].

Section 4 is entirely devoted to ferroelectric and H-bonded systems. It also provides a nice illustration of results that always maintained the utility of proton NMR in solid state, even wideline, or how the old question of the order disorder or displacive nature of some ferroelectric phase transitions were reopened by progresses in NMR resolution. A number of structural phase transition is discontinuous, but the examples of coexistence in solid-state and kinetic studies are rather scarce this is the object of Section 5. Section 6 is devoted to single-crystal studies that allow very precise comprehension of subtle phase transition mechanisms. Section 7 introduces the salient features of NQR that represent an interesting alternative to NMR in some cases. The section ends with a table of miscellaneous phase transitions that complete the references given in the text. Section 8 concludes and presents some perspectives in NMR phase transition studies. 

Despite a number of indications of transitions that cannot be classified within one pure type, strong experimental evidences were usually lacking to support the existence of mixed-type phase transitions. The examples discussed below show that recent progresses in NMR resolution and sensitivity,... 

NMR spectra were usually measured with the sample tube spinning at 20Hz to further improve the NMR resolution. This can introduce signal sidebands at the spiiming speed and its harmonics, and... 

Diffusion ordered projection spectroscopy (DOPY) H/ H DOSY NMR Resolution improvement of correlated signals [123]... 

The inlet and outlet sections of the cell were tapered to prevent eddies which was not only important from the point of view of minimizing chromatographic peak dispersion but also to maintain satisfactory NMR resolution. The volume of the coil region of the tube was 120 ixl which, as seen from Figure 3B, only constitutes part of the total cell volume and consequently is far too great to realize the high resolving power of modern LC columns. 

This cell provided an increased NMR resolution of 2-4 Hz and reduced the sample volume to 10-25 il which is stiU very large in comparison with those normally employed in analytical LC. A set of NMR spectra obtained from the cell is shown in Figure 5. The flow rate was 1 ml/min and the first 15 spectra were taken over a period of 15 sec each (250-pl windows). Spectra 17-32 were taken over 30 sec intervals (OOO-pl windows) and spectra 36 - 40 over 60 sec intervals (1-ml windows). It is clearly seen that even with the improved cell the chromatographic resolution is seriously compromised to obtain NMR sensitivity. Multi-component mixtures that are only just resolved would result in a number of solutes being contained in the sample cell at one time and consequently impair the integrity of the NMR spectrum. Nevertheless, the work of Haw et al. (5), at the time, represented a significant advance in the LC/NMR technique. 

Bayer and his colleagues (6), developed further their original LC/NMR system (1) to provide less band dispersion and greater NMR sensitivity and resolution. An NMR resolution of 0.5... 

Isotopic labelling with deuterium ( H) can be used to provide spectral editing. For example, specific incorporation of a deuterium atom into methylene or methyl groups of amino acids can be used to obtain detailed structural and dynamic information. Dramatic increases in proton NMR resolution for larger proteins can be achieved through random labelling of the protein with deuterium, at levels between 50 and 85%, by growth on substrates in which the ratio of to is controlled. 

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