CPMAS spectra

Figure 9 (A) A, 3C CPMAS spectrum of synthesized A/J42, which was partially...

Figure 2.25 Solid-state 13C NMR CPMAS spectrum of form III of iP4MP.152...

CPMAS spectrum typical of the regular twofold helical conformation (Figure 2.45b).147 191 Samples of sPP which present X-ray powder diffraction patterns typical of the isochiral form II (Figure 2.45c) show a different solid-state 13C NMR spectrum with additional signals in the region of the methylene and methyl carbon resonances (Figure 2A5d). These additional resonances have... 

A, which is shown in the second spectrum from the bottom in Fig. 10.24. Comparing the carbon spectrum of Form A to that of a different crystalline form, Form B, shown in the topmost spectrum of Fig. 10.24, reveals several notable differences in both chemical shifts and line shapes. This, in fact, is often the case for different polymorphs of the same chemical entity, and it enables a clear distinction between the various crystalline forms when present in a mixture. A 13C CPMAS spectrum of a standard sample of the excipients, in the appropriate proportions for the formulation, was also acquired for reference. This is shown as the second spectrum from the top in Fig. 10.24. 

Figure 12.2 (a) 13C CPMAS spectrum of a crushed placebo tablet showing the signals of the excipients only, (b) 13C CPMAS spectrum of a crashed tablet containing 2.5 w/w% unlabeled Org OD 14. The 13C signals of Org OD 14 are only just detectable, (c) 13C CPMAS spectrum of a crashed tablet containing 2.5 w/w% Org OD 14 13C labeled at the 19- and 20-ethynyl and the 21-methyl carbons. The vertical scale in (a) and (b) has been expanded about three times relative to (c) to allow visualization of the unlabeled Org OD 14 13C resonances. 

The variation of the 2 Si CPMAS spectrum was examined as the contact time for the cross polarization experiment was changed. The most intense CP signal was obtained with relatively short contact times, indicative of short silicon-hydrogen internuclear distances. Maximum CP intensity in the -103 ppm peak occurred at about 1 to 2 ms, suggesting to 2gSi distances of a few angstroms or less for this type of silicon. 

Figure 4. (a) Ti MAS spectra for amide, imide, and LiH phases that include a sample made from a mixture of Li2NH and A1 metal reacted at U. Utah with hydrogen gas to form LiNHi and LisAlHe phases [26], (b) Ti H CPMAS spectrum of LiH and LiNHi, where the amide was enriched with the isotope by reacting NHs gas with LiH. Ti CPMAS NMR spectrum of the natural L1NH2 purchased from Aldrich is displaced together for comparison. Note the Ti triplet peaks of the LiNH2 phase are consistent with the three distinct site locations in its crystal structure [28]. 

When applied to catalysts, it can be used to detect, for example, the silicon atoms carrying one or more hydroxyls. Using this technique (usually denoted by the initials CPMAS), it is possible, for silica impregnated by Mg(II) aqueous solutions, to show the presence of surface hydroxyl species on silicon atoms linked to magnesium. In the MAS spectrum of zeolites (cf. Paragr. 13.5.1) the Si(OAl)(OSi)3 have the same chemical shift as Si(OH)(OSi)3. A CPMAS spectrum shows only the signal of the latter. 

Fig. 13. C-CPMAS spectrum of bacterial cellulose containing mainly the la and a minor amount of the Ip polymorphs. Below the calculated chemical shift values according to the chemical shift crystal structure refinement are shown.

Solid-state CPMAS spectrum of the fibroin prepared from cocoons spun by larvae fed with [ C]formate revealed that serine C3 was labelled specifically with C, suggesting that the reverse conversion from glycine into serine took place in the silkworm. " ... 

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. 3. The CPMAS spectrum and four subspectra of cholesteiyl acetate, (a) Standard CPMAS (b) nonprotonated C-dominated subspectrum (c) subspectrum (d) CH2 subspectrum (e) CHs-dominated subspectrum. The numbers indicated in the spectra represent the assignment of the resonance. (Reproduced from Wu et with permission.)...

To ponclude this section, we state that interactions can be recovered simply from high-order effects because the MAS can average only out the first-order terms. When quadrupolar spins are involved, this is practically feasible because the second-order terms become evident. For example, the dipolar information can be extracted from the broadened CPMAS spectrum of a spin-1/2 nucleus bonded to a quadrupolar spin. This phenomenon can date... 

Three forms are present in the commercial solid, evidenced by the three components of methylene and mefliyl carbon resonances. Recrystallization from ethyl acetate results in only one form. Polymorphism was also recognized in the case of caffeine [3232]. The C CPMAS spectrum of solid caffeine (Figure 12-12) shows five carbonyl resonances whereas there are three C=0 groups. The assignment of signals in the spectra of phenobarbital or caffeine is not straightforward, it requires X-ray diffraction structural data and theoretical calculations of shielding constants for the forms present in the crystals. 

Meropenem, a promising agent for combating resistant bacterial infections, is marketed as a powder blend of the crystalline drug and sodium carbonate. NMR spectra recorded for solution and solid state showed [51] that the carbonate in the formulation was the cause of a chemical modification of meropenem in solution. The C CPMAS spectrum of the reconstituted formulation showed the signal of bicarbonate at 159 ppm and a broader one, centered at 164 ppm of meropenem-C02 adduct. It is interesting that such a carbon dioxide adduct could be observed in the solid state, obtained after lyophilization. 

The sensitivity of the - C NMR experiment is limited by the often very long spin-lattice relaxation times of the - C nuclei. Values of T of over 1000 s have been measured in solid poly(ethylene). " It is likely that - C nuclei involved in rigid cross-link structures also possess very long C T relaxation times. As a result pulse repetition times, and hence total scan times are often prohibitively long, especially when the aim is to observe peaks due to low concentrations of products of irradiation. The technique of cross-polarization reduces this problem since the spin temperature of the protons and not the - C nuclei has to reequilibrate before pulse repetition. The H Ti in proton-rich condensed systems is usually much shorter than T. In addition the signal-to-noise in the CPMAS spectrum is increased by a factor of up to four compared with spectra obtained by direct excitation of the spins, due to the larger Boltzmann population of the proton nuclei. 

Figure 12 C-CPMAS spectrum of Gly-Tyr showing disappearance of quadrupolar coupling ( C- N) after reaction with CPAse.

When poly(ethylene oxide) (PEO) powders are y-irradiated under vacuum they are observed [43] to crosslink. The CPMAS/DD and MAS/DD NMR spectra of virgin and vacuum y-irradiated POE (see Figure 5.18) show [44] marked differences. The MAS/DD spectrum of unirradiated PEO shows a well-resolved resonance at 72.36 ppm due to carbons in mobile amorphous regions, which have relatively short values, whilst at 73.43 ppm the CPMAS spectrum shows a weak, broad envelope of resonances due to rigid crystalline carbon environments. The corresponding irradiated PEO sample, on the other hand, presents well-resolved resonances for both crystalline (under CPMAS/DD) and amorphous (under MAS/DD) carbons. Figure 5.18(b) shows that under CP conditions, the crystalline resonances are efficiently cross-... 

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