Scalar decoupling

Fig. 30.—Comparison of C-N.m.r. Spectra of Bovine, Nasal Cartilage in DsO at 37°. (Presented are spectrum A, obtained by using scalar decoupling with yHj/27r = 3.5 kHz, and spectrum B, by using dipolar decoupling of 65 kHz.)...

The linewidths of the high resolution scalar decoupled and NMR have been studied for semicrystalline P(4HB) and P(3HB-co-18%4HB) in the amorphous phase and in the melt as functions of temperature and magnetic field strength [87]. There are two classes of origins, which exert linebroadening of the NMR spectra of amorphous or semicrystalline polymers,... 

Figure 1. 50MHz CP/MAS HMR spectrum of ramie and stick-type scalar-decoupled HMR spectrum of low molecular weight cellulose In DMS0-d5 solution (.8). Broken and thin solid lines In the CP/MAS spectrum are for the crystalline and noncrystalllne components, respectively.

Figure 2. Scalar decoupled (yHt/2ir = 4 kHz) 50.3-MHz C NMR spectra of solid Hytrel and polybutylene terephthalate. Key a, Hytrel 4056 b, Hytrel 5526 c, Hytrel 6346 d, Hytrel 7246 and e, polybutylene terephthalate. All spectra were obtained on the same amount of polymer and were recorded at 34°C with 1024 accumulations and a 3 s pulse repetition rate. Peaks marked with X arise from an internal capillary containing benzene-df...

Figure 3. Line widths of the C NMR (50.3 MHz) solid Hytrel scalar decoupled 29 ppm (O) and 73 ppm resonance fOl as a function of average hard block length at 34°C. The temperature below which the line width increases due to dipolar broadening is slightly different for the two types of aliphatic carbons fsee Figure 4). It also increases slightly as the hard segment content of the polymer increases. The -OCHi- line width for the Hytrel 7246 sample was measured at a temperature below this point and therefore does not fall on this line.

Figure 4. Scalar decoupled line width as a function of temperature for a, Hytrel 7246 and, b, Hytrel 4056. Key O, central -CH - carbons and , -OCHi-...

First, the soft s ment carbons of all Hytrel samples can be observed selectively using scalar decoupling (Figure 2). This indicates that the carbons in the mobde r ons have dynamic properties which are different than those of the ripd carbons. In addition, the T data indicate that the rate of these motions is independent of the hard s ment content of the polymer. If considerable mixed phase were present, one would expect that the number of spins observable with scalar decoupling would be a function of the average hard block length of the polymer. 

As mentioned above, the relaxation phenomena of macromolecules seldom follow the single correlation time theory dictated by eqn (36). In such cases, a wide distribution is usually introduced in the correlation time. However, as discussed elsewhere, the distribution of correlation time not only fails to explain the temperature dependencies of Ti, T2 and the NOE of the non-crystalline components observed by scalar decoupled NMR on linear polyesters and polyethylene, but also overlooks the intrinsic motion of long-chain molecules. On the contrary, the 3r theory dictated by eqn (41) was found to be very effective to describe such temperature dependencies of the relaxation parameters. Irrespectively of whether the motional mode assumed in the 3t model for the C-H vector is really true, the concept that the C-H vector in macromolecules involves plural independent diffusional motions with discretely different correlation times is very useful to explain the magnetic relaxation phenomena of macromolecules, as will be shown later. 

Here some comments should be made in relation to the previously reported scalar decoupled NMR work of LPE samples. The non-crystalline material in these bulk- and solution-crystals can also be detected by the usual scalar decoupled NMR as it is associated with rubbery molecular chain motion. It was found that Ti was almost equivalent for the non-crystalline components of both crystals whereas T2 of the solution-crystals was very much shorter than that of the bulk-crystals. Referring to the above-mentioned results from solid-state NMR, it is assumed that the 31-0 ppm non-crystalline component in the bulk-crystals and the 31-3 ppm non-crystalline overlayer in the solution-crystals were detected by scalar decoupled NMR. 

The simplest means of removing dipolar interactions between two nuclei is to decouple the interaction by a means entirely analogous to scalar decoupling used in NMR spectroscopy to... 

Clearly, there is a decrease (42%) in total intensity in the deoxy relative to the oxygenated form - a result which differs from the haemoglobin A case where the spectra of both oxygenated and deoxygenated forms have the same total intensity under scalar-decoupling conditions. 

In a double-resonance experiment, in addition to the carrier frequency of the C, there is resonant irradiation of the protons that coherently signal-averages the scalar couplings to 0. This is called scalar decoupling. The C resonance is measured at the carbon resonance frequency. Because the scalar couplings are less than... 

For a methyl carbon with scalar-proton decoupling, the intensity of the resulting single line is 24 times greater than the outer lines in the 1 3 3 1 quartet that would be observed without decoupling. For C-NMR spectra, which are recorded for the low natural abundance of carbon, scalar decoupling can be used to obtain better signal-to-noise ratios and to improve the sensitivity. 

The differences in the NMR spectra measured under Fourier Transform conditions with scalar decoupling of the same substance, say water and ice, boggle the mind. The proton NMR spectrum of the water is sharp and narrow with a band width of one Hz, while the proton NMR spectrum of ice is extremely broad with a band width of 20 KHz. This is totally unexpected. In the early days of experimental NMR, the NMR lines of solids were so broad that no measurable signal could be obtained related to chemical structure. 

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