Cross polarization/magic angle spinning spectra

Solid-state C variable-amplitude cross polarization magic-angle spinning (VACP/MAS) nuclear magnetic resonance (NMR) spectra were acquired for the sorbitol samples. Proton decoupling was achieved by a two-pulse phase modulation (TPPM) sequence. Identical C spectra were measured for the y-form sorbitol samples, and a representative spectrum is shown in Figure 9. 

FIGURE 2. The cross-polarization, magic-angle-spinning, solid state C NMR spectrum of stream humic acid from the Ogeechee River near Louisville, Georgia during December 1981. 

C-cross polarization magic angle spinning (CP-MAS) spectra were obtained on a Bruker CXP-100 instrument. A rotor consisting of a barrel of boron nitride and a base of Kel-F was used. Rotor speed was 3.8 kHz. Recycle time was varied from 0.3 to 1 s. A variety of contact times from 0.5 to 3ms were employed. The Hartmann-Hahn condition was set using a sample of hexamethylbenzene. Chemical shifts were measured with respect to external hexamethylbenzene (by storing the hexamethylbenzene spectrum in another computer memory block) but are quoted with respect to TMS. It is assumed that the chemical shifts of hexamethylbenzene with respect to TMS are the same in solution as in the solid state. 

A strong signal at 10 ppm in the solid state cross polarization/magic angle spinning (CP/MAS) NMR spectrum of stone-ground wood pulp (SOW) treated with trimethyl phosphite, has been attributed to a cyclic phosphite ester formed via hydrolysis of an oxyphosphorane adduct of trimethyl phosphite and ort/to-quinones present in lignin, as shown in Scheme 6.2 [64-66]. 

The primary components and the chemical structure of the raw peat and the solid product were further analyzed by Fourier transform infrared spectroscopy (FTIR) 0ASCO 670 Plus) using the Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS) technique and the JASCO IR Mentor Pro 6.5 software for spectral analysis. The cross polarization/magic angle spinning (CP/MAS) NMR spectrum of raw peat and the solid... 

Until recently, insolubility prevented the use of NMR spectroscopy as a routine tool for PPP characterization. In 1980, the first solid-state NMR spectrum ( C) of PPP was obtained by use of cross-polarization magic-angle spinning (CP/MAS) techniques [201]. Since that time other C NMR and H NMR studies have been reported [202,203]. Furthermore, the development of the metal-catalyzed route to substituted PPPs has made it possible to use this technique routinely, for structural characterization in solution [73-82]. 

The insoluble nature of suberin prevents its analysis by conventional NMR spectroscopy. The development of solid sample C-NMR spectroscopy with cross polarization/magic-angle spinning provides an opportunity to obtain spectra of the intact polymer. The NMR spectrum of the cutin polymer from the fruit of Malus pumila indicates, as expected, that the majority of carbons are methylenes (CH2, 20-30 ppm). The spectrum of suberin from 5. tuberosum showed the presence of a high proportion of aliphatic CH2 but also had a large amount of CHOH (60 to 80 ppm) carbon, probably from contaminating cell wall carbohydrates. This presumed carbohydrate material was still present in the residue left after LiAlH4 reduction, whereas the majority of aliphatic CH2 had been removed by this treatment (Fig. 6.4.8). This result shows that not much of the... 

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