Broad line NMR

Benzene-ring orientation 100 Boundary layer 151 Broad line NMR 197 Buckling of fibrils 140... 

Al MAS NMR has been demonstrated to be an invaluable tool for the zeoHte sdentist It provides a simple and direct way to quantify the proportions of A1 in four [Al(4)j, five [Al(5)j and six [Al(6)j coordinations. Quantitative determination of these species is an important issue in catalysis, and major effort is devoted on this topic. As mentioned already, for A1 only the central transition (-i-half to —half selective exdtation ) is detected. The central transition is unaffected by first order quadmpolar interaction, but the presence of second order effects causes broadening and complicates the quantitation of the A1 species. Usually hydrated samples and short radiofrequency pulses are employed for quantitative determination of framework and extra framework aluminum species. It is uncertain whether hydration changes the coordination of A1 species. Certain extra framework A1 can have very large quadmpolar interactions resulting in very broad lines ( NMR invisible ) [155, 202]. Unlike Si NMR, Al has a short relaxation time due to its quadmpolar nature, and the Al NMR spectrum with good signal to noise can be obtained in a relatively short time. 

Broad-line NMR derivative spectra were obtained using a Brucker HFX-90 spectrometer to record the resonance at 84.67 MHz. The specimens, made by compacting granular PTFE into preforms, sintering at 380°C, and cooling slowly at a rate of 0.02 deg/min had a specific gravity of 2.205. The second moment of tire NMR line shape is of interest because the fourth moment of the orientation distribution function is proportional to it. 

Hand-shaken dispersions of phospholipids thus may be a more reasonable choice for comparison with membranes, and except in rare cases of highly unsaturated systems the high resolution NMR spectra of such dispersions do not exhibit high resolution proton absorption. Hence, for most membranes, NMR can indicate immobilization of fatty acid chains but not whether chains are immobilized by binding onto proteins or by tight packing into bilayers. Broad line NMR may be useful, especially when a phase transition is present. 

Figure 15. Broad line proton NMR spectrum of total lipids from bovine erythrocytes, dispersed in D20 without sonication (15 w/v%). Spectrum was taken at 30° C. with a Varian V-4200 broad line NMR...

Broad-line NMR, 359 6-type transition, 219 Butadiene excited states, 312-313... 

Up to here, we have considered only the interaction between a proton and the F19 nucleus bonded to it. There are also weaker interactions between a given proton and more-distant nuclei in the crystal. These interactions superimpose doublets with smaller splittings on the main proton doublet. The net effect is a doublet whose lines are greatly broadened. For this reason, NMR studies of solids are called broad-line NMR. Because of this broadening and the smallness of chemical-shift differences as compared to spin-spin splittings, different resonance lines due to chemically nonequivalent protons are not observed in NMR spectroscopy of solids (unless special techniques are used). 

In the early stages of the development of NMR techniques (1950s-mid 1970s), the studies of polymers could be classified into two major domains broad line NMR of the solid state and the high resolution NMR of polymer liquids and solutions. In this period, crosslinked polymers were investigated by the broad line and pulsed NMR techniques, respectively. These studies in the solid state yield information primarily on macromolecular dynamics, and indirectly on the network structure. 

Two types of low resolution solid state NMR techniques can be distinguished a) broad line NMR in which the absorption signal is obtained by sweeping the magnetic induction B in the vicinity of the resonance value B0 (Eq. (1)), and b) pulsed techniques which are based on the possibility of rotating the magnetization under the influence of particular radiofrequency pulses, or pulse sequences. 

Variations in broad line NMR are primarily sensitive to motion. Consequently, changes in Av and AM2 are indications of motional processes in the system. If, as a first approximation, the relaxation process can be expressed by a single correlation time, xc, the following relation between NMR parameters and tc can be established ... 

Table 1. Apparent activation energies for relaxation processes in crossEnked polymers calculated from broad line NMR...

Fig. 14. Dependence of the relaxation times T2. and the fractions of protons with different mobility (f.) for unsaturated polyester on the curing time, as measured from broad line NMR ( ), Hahn spin-echo ( ) and Carr-Purcell pulse sequence (O)- Symbol x indicates the initial distribution of styrene and unsaturated polyester protons (adapted from Ref. S5))...

These equations can be also expressed as functions of the strength H of the main magnetic field by using a relation of = yH, were y is the nuclear gyromagnetic ratio. Such expressions may be more useful for the usual broad-line NMR spectrometry in which the main field is slowly swept under a constant rotating subfield. 

As pointed out in Chapter I, the NMR absorption spectrum for polymers in the solid state is generally very broad. In such cases the NMR is usually observed as the so-called broad-line spectrum. In this, the resonance is recorded by slowly sweeping the main static magnetic field H modulated with a small amplitude and frequency under a constant high-frequency subfield rotating perpendicularly to the main field. Figure 5 shows schematically the principle of the measurement for the broad-line NMR... 

Mandelkern, L., Chow, C. Y. private communication Chow, C. Y. A broad-line NMR study of molecular weight fractions of bulk polyethylene. Ph. D. dissertation, Florida State University 1973... 

Until recently only broad-line NMR has been used for observing the 23Na resonance. The technique is not well suited to obtaining absolute intensities. 

Molecular Weight Dependence of Phase Structure. Similar line shape analysis was performed for samples with molecular weight over a very wide range that had been crystallized from the melt. In some samples, an additional crystalline line appears at 34.4 ppm which can be assigned to trans-trans methylene sequences in a monoclinic crystal form. Therefore the spectrum was analyzed in terms of four Lorentzian functions with different peak positions and line widths i.e. for two crystalline and two noncrystalline lines. Reasonable curve fitting was also obtained in these cases. The results are plotted by solid circles on the data of the broad-line H NMR in Fig. 3. The mass fractions of the crystalline, amorphous phases and the crystalline-amorphous interphase are in good accord with those of the broad, narrow, and intermediate components from the broad-line NMR analysis. 

The study of the phase structure of semicrystalline polymers, particularly that of the interfacial region, is of foremost importance, since it has an intimate relationship with the macroscopic properties of polymers. Many techniques have been used to study this problem. To determine the mass fraction of each phase of the three-phase structure, such techniques as H broad-line NMR or Raman... 

Use of non-standard abbreviations might let the paper go unnoticed, as the abbreviation in the title or in the text might not be recognized by automatic indexing services (e.g. MASS NMR instead of the common MAS NMR or MASNMR similarly, SSNMR for Solid-State NMR could mean high-resolution or broad-line NMR). 

Besides NQR spectroscopy and the study of nuclear quadrupole interaction effects in broad-line NMR spectroscopy, paramagnetic electron resonance 6°1, Mossbauer spectroscopy, and the study of perturbed angular correlation of y-rays, are suitable methods for studying nuclear quadrupole interactions in solids. Indirect methods are also available for acquiring information about the nuclear quadrupole couplinjg constant from the liquid state (particularly NMR spectroscopy in liquids and in liquid crystals in some cases gives information about this constant). By microwave spectroscopy, the nuclear quadrupole interaction may be studied in the gaseous phase (see the paper by Zeil). We shall deal here only with the aspect of NQR spectroscopy in solids since this method has the broadest applicability to chemical problems in comparison with the other methods mentioned. 

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