Wide-line spectra

We shall treat the heteronuclear and homonuclear systems separately. Although there are many features in common in the (wide line) spectra produced, the differences in the form of the Hamiltonian suggest quite different methods for manipulating the spin systems so as to produce narrow lines and high resolution spectra. 

Table 4 shows the results obtained by the H wide line NMR method. For coal molecular structure, a two-phase model is widely accepted which comprises a high mobility component and a low mobility component.103 Therefore, the wide line spectra have been analysed using a deconvolution technique that separates spectra into mobile and immobile components. It is clear that the amount of mobile component is increased in the case of both rapid and slow heating treatments. The rapid heating treatment in particular has a large effect on the increase of the mobile component because of the changes to the non-covalent bond network structure and the molecular structure. 

Since the spin-spin relaxation mechanism in solids is so efficient, very small values of Ti result, producing very broad lines. Further, such spin exchanges between identical nuclei average the resonance frequencies of nuclei whose environments are not quite identical, and broaden the spectral lines further (dipolar broadening). For these reasons, solids give spectra with lines about 1000 times too broad to give information of much chemical interest (see below). Spectra of solids, or wide-line spectra, will therefore not be further discussed here, although they are of interest in solid-state physics. 

As already stated, rapid reorientation of molecules is a necessary prerequisite for the observation of high-resolution NMR spectra solids give rise to wide-line spectra of little chemical interest. However, at intermediate rates of molecular reorientation, it is possible to obtain high-resolution NMR spectra without averaging out through-space interactions between magnetic nuclei. 

Wide-line spectra are observed for solids, or for systems In solution with very long (with respect to the Inverse of the quadrupole splitting) correlation times for re-orlentatlon. Examples of the latter are collagen fibres, membranes, and liquid crystals. As many examples of this type of study were given In our earlier review (2), we shall confine ourselves to collagen and membranes. 

Other polymers whose phase structure has been studied are polypropylene, poly(tetrahydrofuran), and polyurethanes. The detection of four components in the wide-line spectrum of polypropylene fibres was claimed, the narrowest being attributed to absorbed water. In the study on poly(tetrahydrofuran), the molecular orientation in drawn and rolled sheets was examined. The anisotropy of the H wide-line spectra were well-explained by uniaxiai and double orientations of crystallites. Assink observed two-component free induction decays in polyurethanes at room temperature, attributed to glassy and rubbery constituents. Immediately after quenching from 170 °C, a linear polyurethane showed a continuum of domain compositions, but after a few hours, the original two-phase structure was re-established. 

Wide-line spectra are those in which the bandwidth of the source of the lines is large enough that the fine structure due to chemical environment is obscured. Figure 19-11 is a wide-line spectrum for a mixture of several isotopes. A single resonance is associated with each species. Wide-line spectra are useful for the quantitative determination of isotopes and for studies of the physical environment of the absorbing species. Wide-line spectra are usually obtained at relatively low magnetic field strength. 

A useful method to correlate IH and 13C signals is wide-line separation In a two-dimensional (2D) spectrum high-resolution 13C CP MAS spectra are taken in one dimension, and correlated to the IH wide-line spectra of the protons attached to each resolvable carbon atom. This enables the correlation between the IH mobility, as detected by the linewidth, and the 13C structure viewed by the chemical shifts. 

In practice, the situation can be more complicated, since motions around interdependent axes might lead to complex motional patterns, or the EFG may have no axial symmetry. The resulting 2H wide/line spectrum then can exhibit a modified shape, from which further information about the geometry of motions can be obtained. 2H wide-line spectra thus contain a wealth of information about the time scales and geometries of local bond motions in systems with partial motional averaging. Order parameters are established in characterising liquid crystalline phases, particularly for lipids. Typical values of S in aggregated alkyl chains of volume phases are on the order of 0.1- 0.2. 

The principle of fast and slow motional averaging (also described in the next chapter) applies similarly to protons and dipolar interactions. IH wide-line spectra are generally without structure, due to the superposition of a large number of dipolar interactions. This makes 2H wide-line investigations far more attractive than those of protons in spite of the lower sensitivity and necessity for isotopic labelling. 

Anderson et al. have presented solid-state NMR analysis of local structural environments in phosphate glasses for educational purposes. " In particular, the solid-state NMR wide-line spectra of a series of sodium phosphate glasses have been considered, which can also be simulated by spectral addition of reference solid-state spectra obtained for pure pyrophosphate and metaphosphate salts. The example chosen introduces the principles of solid-state NMR and allows interpretation of the spectrum in terms of the composition and localised phosphate environment. 

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