NMR signals of solids

Differences in the. solid-state NMR signals of crystalline forms having identical conformations have been also observed. For instance, well-crystallized a form samples of i-PP show splittings for the methyl (22.6, 22.1 ppm) and methylene resonances (45.2, 44.2 ppm) into two lines with relative intensities 2 1 [117,118]. These splittings have been interpreted in terms of the known crystalline packing of the a form, which is characterized by pairs of 3/1 helices of opposite handedness at closer distances (Fig. 10). This generates inequivalence between the carbons indicated as A and those indicated as B in Fig. 10 [117,118]. 

Fig. 12. Tentative structure of lignin according to Adler together with typical solid state NMR signals of its carbon atoms...

The solid state 13C NMR signals of OE in its complexes with a-CD appeared separately from the spectra of a-CD-PEG complexes in a higher magnetic field, in which the signals of the two components were overlapped. This... 

The surface of a solid may be considered as a defect of the solid. Nuclei near the surface of the solid will experience large electric field gradients due to the asymmetry of their environment. If these gradients are suflSciently large, the NMR signal of these nuclei in a polycrystalline sample will be broadened enough so as not to be detected by the instrument. The fraction F of aluminum nuclei so situated will not contribute to the observed intensity 1.1 is related to F in the following manner... 

Fig. 9 Transverse relaxations times T2 of the Rb NMR signals of the two coexisting phase states versus temperature in D-RADP-25 resulting from the fit of Eq. 3 to the 2D time frequency spectra. The solid lines are guide lines for the eye...

R. Siegel, T. T. Nakashima and R. E. Wasylishen, Signal-to-noise enhancement of NMR spectra of solids using multiple-pulse spin-echo experiments. Concepts Magn. Reson., 2005, 26A, 62-77. 

When a solid sample of A-acetylproline is dissolved at — 60°C in CD3OD, the H-NMR signals of only one species are observed (72CC788) whereas if the sample is allowed to reach room temperature, signals from a second con-... 

A suitable way to enhance NMR signals of nuclei with small magnetogyric ratios or low concentrations (rare spins 5) interacting with abundant spins I is the polarization transfer from the spin I to the spin S ensemble via a cross polarization (CP) experiment. This experiment is based on a double resonance technique, which can be applied in combination with MAS for the characterization of surface sites of solid catalysts and rigidly bound surface complexes. The pulse sequence used for CP experiments is demonstrated in Fig. 4. 

The two signal 15N NMR spectrum of solid imidazole indicates that tautomerism is slow in the solid state. The average (186 ppm) is the same as the shift observed in solution. As the solution pH decreases the 15N signal moves progressively upheld until both nitrogens are protonated (82JA1192). 

---