13C CP MAS spectra

This is illustrated in Figure 10 which shows the 13C CP/MAS spectrum of the fluxional organometallic cyclooctatetraene-diiron pentacarbonyl (4) (13), whose localized1 bonding scheme determined by diffraction measurements is shown inset in the figure. 

Figure 10. 13C-CP/MAS spectrum at 22.6 MHz (90 MHz 1H) of cyclooctate-traenediiron pentacarbonyl (4) shown along with the localized bonding scheme suggested from x-ray diffraction measurements.

FIGURE 38. (a) 13C CP/MAS spectrum of THY after 56 h. The proposed structure from this spectrum is the ladder polymer 101 shown in Figure 37. (b) The spectmm after 11 days. The proposed stmcture from this spectmm is the ladder polymer 102 shown in Figure 37. Reprinted with permission from Reference 53. Copyright (1994) American Chemical Society... 

We are concerned with the conformational characterization of a PDA/PLV blend sample. The observed 13C CP/MAS NMR spectrum for the PDA/PLV (50/50) blend sample is shown in Fig. 12 (b). In the 13C CP/MAS spectrum, a new peak for the Ca carbon of PDA appears clearly at 48.9 ppm. This peak can be assigned to the Ca carbon of PDA in the p-sheet form. In order to clarify in detail the appearance of this new peak, the carbonyl-carbon region and the Ca, Cp and Cy carbon regions in the spectrum of the PDA/PLV (50/50) blend sample were expanded as shown in Fig. 13. By computer-fitting the observed spectrum was decomposed, and then the fractions of the a-helix and p-sheet forms for PDA and PLV were determined. Another new peak of the Cp carbon of PDA appears at about 20.9 ppm, in addition to an intense peak assigned to the a-helix form (15.7 ppm), and this can be assigned to the p-sheet form (20.9 ppm). These results show that the a-helix form of PDA in the PDA/PLV blends is partially transformed. 

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. 

The 13C-CP-MAS spectrum has also been used to complement the diffraction data relating to the crystal structure of cellulose, but hitherto the results have been inconclusive [242]. A significant amount of H single crystal spectroscopy has been reported and has been used as a basis for correlation with neutron diffraction data and with theoretical ab-initio molecular orbital calculations [243, 244]. 

An advantage of the heteronuclear H-13C approach is that it benefits from the better resolution in a 13C SQ dimension. An example of this is provided by the hexa(joara-n-dodecylphenyl)-substituted HBC (henceforth referred to as HBC—PhCi2), 7.37 In this case, H solid-state NMR is unable to distinguish the core and exo-phenyl protons. By comparison, as shown in the 13C CP MAS spectrum at the top right of Figure... 

Fig. 8. The 13C CP MAS spectrum of naphthylphenylsulphoxide, showing the region containing the phenyl resonances, as a function of temperature (left) and the simulations of these experimental lineshapes (right) assuming restricted rotation of the phenyl ring about the S-Ph bond with rate constant k.13...

Fig. 12.13C-NMR spectrum of erythrodiisotactic poly(l,2-dimethyltetramethylene) at 75.47 MHz and 303 K. a) in solution of CDC13, b) CP-MAS spectrum of the semicrystalline polymer in the bulk. Chemical shifts given at the signals refer to TMS = 0 ppm. (Ref.20))...

Figure 3. 13C-CP/MAS solid-state spectrum of the cation shown illustrating the resolution obtainable from crystalline samples.

Fig. 9 Examples of simplifying solid state NMR spectra by the TOSS and delayed decoupling pulse sequences. Shown is a comparison of the 31P CP/MAS NMR spectrum of fosinopril sodium utilizing the standard pulse sequence (A) and the TOSS routine (B). Also shown is the full 13C CP/MAS NMR spectrum of fosinopril sodium (C) and the nonprotonated carbon spectrum (D) obtained from the delayed decoupling pulse sequence utilizing a 80 /us delay time. Signals due to the methyl carbon resonances (0-30 ppm) are not completely eliminated due to the rapid methyl group rotation, which reduces the carbon-proton dipolar couplings.

Quantitative solid state 13C CP/MAS NMR has been used to determine the relative amounts of carbamazepine anhydrate and carbamazepine dihydrate in mixtures [59]. The 13C NMR spectra for the two forms did not appear different, although sufficient S/N for the spectrum of the anhydrous form required long accumulation times. This was determined to be due to the slow proton relaxation rate for this form. Utilizing the fact that different proton spin-lattice relaxation times exist for the two different pseudopolymorphic forms, a quantitative method was developed. The dihydrate form displayed a relatively short relaxation time, permitting interpulse delay times of only 10 seconds to obtain full-intensity spectra of the dihydrate form while displaying no signal due to the anhydrous... 

The 13C CP/MAS NMR spectrum taken 30 min after recrystallization of THY is shown in Figure 36A, and the 13C chemical shift values of this spectrum together with those of the THY monomer in CDCI3 solution are summarized in Table 35. 

The observed 13C CP/MAS NMR spectra of pure PLA and pure PLV are shown in Fig. 3 (a) and (d), respectively. The assignments of these spectra are straightforwardly made by using reference data of polypeptides with the a-helix and p-sheet forms reported previously.21,22,26 The 13C chemical shift values of these polypeptide samples are listed together with reference data of PLA and PLV with the right-handed a-helix form and the p-sheet form in Table 3. The three intense peaks at 177.0, 53.2, and 15.8 ppm which appear in the spectrum of pure PLA (Fig. 3 (a)) can be assigned to the C=0, Ca and Cp carbons, respectively. From these 13C chemical shift values, it is found that PLA takes the right-handed a-helix form. There are no peaks which come from the P-sheet form. On the other hand, in the spectrum of PLV (Fig. 3 (d)), the four intense peaks appear at 172.4, 58.9, 33.2 and... 

Fyfe et al. (355) were able to produce a very informative 13C CP/MAS NMR spectrum of the triphenylmethyl carbonium ion by using the tetra-fluoroborate counterion and by employing simultaneous 19F and H decoupling during spectral acquisition (see Fig. 81). The nonequivalence of the ortho and meta carbons is readily seen in the spectrum. 

Figure 7. 13CP alanine shielding tensor results. A, 11 Tesla 13C CP-MAS NMR spectrum of crystalline ala-[l3C]ala-ala,vr =1.2 kHz. B, graph showing experimental shift tensor elements versus computed shieldings slope = 1.07, R2 = 0.99. 

Figure 9.6 Hydrated silica, modified in toluene solvent, air cured, (a) 29Si CP MAS NMR spectrum (b) 13C CP MAS NMR spectrum.

Fedorov and Rebrov97 studied the 13C CP/MAS NMR spectrum of dithiazone and compared the data obtained with those from x-ray studies. 

Finally, three of the four benzo-ring aromatic methine carbon signals are resolved in the 7) cp/mas experiment (d) 136.71, 129.16, and 130.3(4), and their respective 7) values are (s) 111, 203, and 271. These solid-state 7) values are consistent with ca. 2.9, 3.8, and 4.5 A distances from the nearest methyl group proton, and enable their respective assignments to be C(7), C(8), and C(9). Now that most of the 13C cp/mas chemical shifts have been assigned, the close correspondence of these values to those in the solution-state spectrum provided further evidence (in addition to the NOE results noted above) that the predominant conformation in solution is (reference,S)-BB. 

Selected solution-state 1H- and 13C-NMR, and solid-state cp/mas 13C parameters have been listed in Table 9. It is seen that some of the aliphatic carbons (and some of the aromatic also, not listed) gave double signals in the cp/mas spectrum. H(31) was assigned as trans to H(41) (they are on opposite faces of the ring), and H(32) was assigned as trans to H(42) on the basis of the magnitudes of their respective 6.4 Hz 3/(31 11) and 8.6 Hz 3.7(32-42) values. In addition, a 6.9% NOE intensity... 

The solid-state 13C cp/mas for crystalline 39, and the quantum mechanical calculated 13C chemical shifts and JHh coupling constants for a TBC model are listed in Table 15. Comparison of the 13C chemical shift values in the cp/mas spectrum versus corresponding values for those from the quantum mechanical calculation shows them to be similar. Dissolution of either quaternary ammonium salt in CDC13 solution affords two species (88 12) in which the major species has undergone a conformational change to a (reference,S)-TCBtype3 structure, while the... 

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