Dipolar dephased spectra

Figure 2 REDOR-based dipolar dephasing spectra obtained for HAp (left) and OCP (right). The solid and dashed lines represent the MAS spectra obtained with and without dipolar dephasing. In other words, any 31P signals retained after the dipolar dephasing must arise from species remote from protons.

The appropriate C6D5 deuterated isotopomers of 28, 29 and 32 were also studied. C—D carbons in the deuterated isopotomers behave in dipolar dephased spectra as quaternary carbons and the changes in spectral patterns of nondeuterated and pentadeuterated compounds were used for 13C chemical shift assignment. The spectra of 28, 30, 31 and 32 were also recorded at elevated temperature. It was found, contrary to Maciejewska s results,87 that in 28, 30 and 32 rotation of the aromatic rings is induced at elevated temperatures. 

Typical dipolar dephased spectra of a sedimentary sea loch humic acid are shown in Figure 4 as a function of dephasing time. 

Figure 9. Dipolar dephased spectra of intact sediments, BX-3.

The peak at 150 ppm is due to the isocyanurate carbonyl carbon, the benzylic substituted aromatic carbons para- to an isocyanurate moiety are shown by a peak at 145 ppm, the 130 ppm peak is due to the protonated aromatic carbons, the shoulder at 125 ppm is due to both isocyanate and ortho aromatic carbons. These signals were confirmed by using dipolar dephasing spectra. However, because of the complexity of the spectra CP/MAS NMR was used to clarify the structures present at the different cure temperatures. A summary of the moieties found in the resin is given in Fig. 15.2.15. 

FIGURE 34.15 CRAMPS dipolar-dephasing spectra of three... 

Fig. 16 CP/MAS NMR spectra of Cd(mtn)Ni(CN)4.1/2QHi2 as a function of temperature, showing fade-out of the cyclohexanol resonances when the molecule reorients at rates comparable to the strength of the decoupling field [53]. Normal spectra are shown on the left, and dipolar dephased spectra, where resonances of carbons strongly coupled to protons are much weaker, on the right.

Figure 5. Effect of dipolar dephaslng on signal Intensities from various carbons in (a), (b) peat and (c), (d) decayed pine leaves. The methoxy (55 ppm) and methyl (20 ppm) signals in conventional spectrum (a) are retained after 40 ps dipolar dephasing (spectrum b). Also acetal resonance (102 ppm) (spectrum a) is lost after dipolar dephasing in (spectrum b). In decayed pine leaf spectra, ketal resonance (105 ppm) in conventional spectrum (c) is retained after 40 ps dipolar dephasing, spectrum (d). (Reproduced with permission from reference 6. Copyright 1983 Pergamon Press.)...

In chemical shift calculations for acylium ions, it was not necessary to model the ionic lattice to obtain accurate values. These ions have tetravalent carbons with no formally empty orbitals, as verified by natural bond orbital calculations (89). Shift calculations for simple carbenium ions with formally empty orbitals may require treatment of the medium. We prepared the isopropyl cation by the adsorption of 2-bromopropane-2-13C onto frozen SbF5 at 223 K and obtained a 13C CP/MAS spectrum at 83 K (53). Analysis of the spinning sidebands yielded experimental values of = 497 ppm, 822 = 385 ppm, and (%3 = 77 ppm. The isotropic 13C shift, 320 ppm, is within 1 ppm of the value in magic acid solution (17). Other NMR evidence includes dipolar dephasing experiments and observation at higher temperature of a scalar doublet ( c-h = 165 Hz) for the cation center. 

Fig. 47. 15 MHz 13C MAS NMR spectrum of cured furfuryl alcohol resin a. The spectrum of the same resin obtained with dipolar dephasing (100 ps interrupted decoupling) b (reprinted from Ref.2341 with permission)...

Assignment based on GIAO 30 values calculated from a BB-conformation DFT/B3LYP model. 7Cqllat or Cmethyi observed in 60 ps dipolar dephasing delay NQS spectrum. 

Fig. 18a, b Spectra of the CAVl 1/DMF crystals a CP/MAS spectrum recorded with 4 ms contact time. Asterisks denote spinning sidebands b dipolar-dephased CP/MAS spectrum. Peaks C2 and C5 are absent from the spectrum while C7 and CH2 peaks are still visible, probably due to segmental mobility of the aliphatic chains. (Adopted from [55] with permission)... 

The difference of relaxation times in different domains makes it possible to observe the spectrum of one of the domains. Figure 10.23(a), shows the Ti-selected spectrum of PVPh/PEO = 40/60 [34]. Since the Ti of crystalline PEO ( 15 s) is much longer than that of the amorphous phase (—0.1 s), it is possible to observe the spectrum of crystalline PEO selectively (indicated by arrow in Fig. 10.23(a)). On the other hand, for the miscible PVPh-rich blend (PVPh/PEO = 58/42), the crystalline-PEO peak is not appreciable. This is in agreement with the above-mentioned results (Table 10.2). The signals of mobile domains/component polymers can be observed selectively by utilizing the weaker dipolar interaction between H. To name a few examples, the dipolar dephasing [128,131,152], the cross-polarization-depolarization [152] and the pulse saturation transfer [151] techniques have been applied. 

O Donnell and Whittaker [51] reassigned the C CPMAS spectrum of Kapton, in part after consideration of the results of the dipolar dephasing experiment, and of the relative peak intensities in the NMR spectra. In addition, spectra were also obtained of solutions of Kapton in concentrated sulfuric acid. Several of the peaks in the high-quality spectra were split into doublets, which was ascribed to the presence of two different rotational conformers having equal energy. 

Fig. 15.2.37. Part of a " C CP/MAS spectrum of a solid polyester obtained (a) without dipolar dephasing and (b) with dipolar dephasing. Chemical shift are shown by the numbers.

Figure 28 39.75 MHz " Si CP-MAS NMR spectra of Fisher S-679 silica gel obtained with six different H- H dipolar-dephasing times (as indicated) prior to H- " Si crosspolarization. Cross-polarization time, 100 xs. Each spectrum is the result of 20,000 accumulations. Spectra in the left column have been scaled to the same peak height for the — 99 ppm peak, and spectra in the right column are plotted on the same absolute scale. (From Ref. 60.)...

FIGURE 34.20 Si CP-MAS NMR spectra of Fisher S-679 silica gel obtained with 2 xs (top spectrum of each set) and two rotor periods (1.04 ms bottom of each set) of H- H dipolar-dephasing prior to four different H- Si CP contact times. MAS speed, 1.9 kHz, (a) cp = 100 fxs (top spectrum, 7376 accumulations bottom spectrum, 82,504 accumulations), (b) tcp = 300 JLS (top spectrum, 2400 accumulations bottom spectrum, 46,200 accumulations), (c) tcp = 1 ms (top spectrum, 432 accumulations bottom spectrum, 21,232 accumulations), (d) tcp = 5 ms (top spectrum, 600 accumulations bottom spectrum, 8320 accumulations). Taken from reference 3a. With permission. 

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