Dipolar-dephasing experiments

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. 

Smernik, R. J., and Oades, J. M. (2001). Solid-state 13C NMR dipolar dephasing experiments for quantifying protonated and non-protonated carbon in soil organic matter and model systems. Ear. J. Soil Sci. 52,103-120. 

Figure 4. Contour plot for 2-D dipolar dephasing experiment.

The nonequivalence of some signals that are equivalent in solution NMR complicates the interpretation of the 13C CP/MAS NMR spectra of azo dyes, even for simple compounds where the chemical shifts measured in solution and their interpretation are available. The dipolar dephasing experiment permits the selective measurement of nonprotonated carbon (i.e. with no directly bound hydrogen). We have used Q,Ds deuterated isotopomers88 in the analysis of the 13C CP/MAS spectra of azo dyes. I3C CP/MAS NMR spectra of 4-[/V,/V-bis(2-hydroxyethyl)amino]azobenzene (28), 2-hydroxy-5-ter -butylazo-benzene (29) 4-(Ar,A-dimethylamino)azobenzene (30), 4-methoxyazobenzene (31) and 4-hydroxybenzene (32) were recorded. 

Olivieri et al92 have reported the 13C CP/MAS NMR spectra of substituted l-phenylazo-2-naphthol derivatives using standard and dipolar-dephased experiments and compared the structural results with solution NMR and x-ray data. 

Dipolar dephasing experiments were performed by inserting a delay before data acquisition in which the decoupler was gated off. 

Table II illustrates the types of structures which may be distinguished from each other by dipolar dephasing experiments on humic substances. Clearly, methine and methyl, protonated aromatic and non-protonated aromatic, ketone and aldehyde, ketal and acetal carbons and also protonated olefinic and non-protonated olefinic carbon can be distinguished. Examples of the use of the method (, 13), are shown in Figure 5.

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. 

Murphy et al. [73] studied the cure and degradation of an acetylene-terminated N-labelled poly(imide) using N CPMAS NMR. Initially, the conversion of the amic acid to the imide precursor was followed. Four resolved peaks are observed due to amide and imide either attached to a phenyl ring or at the terminal position. Measurements of the rate of crosspolarization, and the dipolar dephasing experiment, assisted in the assignment to the spectra. Very different rates of cross-polarization (1/Tnh), and values of Ti, were measured for the various structures. Imidization was incomplete after heating to 670 K for 1 h, a result at variance with the results of... 

Dipolar dephasing. dipolar-dephasing experiments are designed to... 

However, this model is not entirely consistent with the CRAMPS-determined H dipolar-dephasing behavior (39), the main basis for distinguishing between the clustered and isolated protons in H CRAMPS spectra. This model is also inconsistent with related H spin-diffusion behavior, as reflected in some preliminary 1H-29Si dipolar-dephasing experiments, 29Si-detected H spin-diffusion results, and 29Si CP-MAS spectra detected in the absence of H decoupling. Such experiments,... 

Figure 16 Dipolar-dephasing experiment (a) pulse sequence (b) results for untreated silica gel (showing t values) (c) results for sample evacuated at 200°C (showing t values). (From Ref. 43.)...

The solid state NMR measurements on the hard coke concentrates were carried out at 25 MHz on a Bruker DSX spectrometer with MAS at 4.5-5.0 kHz to give spectra in which the sideband intensities are only ca. 6-7% of the central aromatic bands. A contact time of 1 ms was used for the cross polarisation (CP) measurements and the H decoupling and spin-lock field was ca. 60 kHz. The FIDs were processed using a Lorentzian linebroadening factor of 50 Hz. To determine the fraction of protonated and non-protonated carbon, four delay periods between 1 and 100 is were employed in dipolar dephasing experiments. 

Fig. 6. Schematic representation of the Gaussian-Lorentzian two-component decay in a dipolar dephasing experiment.

FIGURE 34.11 CRAMPS dipolar-dephasing experiment, (a) Results for partially dried (in dry box for 2 h) silica gel (showing T values), (b) Results for sample evacuated at 200°C (showing rvalues). Taken from reference 4c. With permission. 

Another Si CP-MAS experiment useful for correlat-ing H and Si spin behaviors is the H— Si analog of the common — C dipolar-dephasing experiment. In... 

---