Mobility chemical shift anisotropy

It can be seen in Fig. 14 that, as the temperature increases, there is a decrease in the chemical shift anisotropy (<733-an) of protonated carbons, indicating an increase in the mobility of the aromatic rings at higher tern-... 

Phosphorus-31 chemical shift anisotropies for trimethylphosphine, its oxide, and its sulphide were 6, +210, and +127 p.p.m., respectively. jr-Bonding will provide a cylindrical mobile electron cloud which can circulate freely when the symmetry axis is parallel to the magnetic field, but which is hindered when it is perpendicular. Thus Cii — cTj should be positive, as is observed for the oxide and sulphide, and the smaller value for the sulphide compared to the oxide could reflect reduced n-bonding. 22... 

Fig. 24. Experimental and simulated ( H)- C cross-polarization spectra of poly-5-caprolactone nanospheres in aqueous suspension after 12 h freezing. The calculated spectrum reproduces the carbon spectmm of four different methylene groups based on the chemical shift anisotropy listed in Table 1. It represents a superposition of two fractions of different isotropic mobility (80% T=0.1 ms 20% r=0.007 ms).

All our attempts to observe Pt NMR signals from either PtCli " or cis-Pt( NH3)aCl2 bound to reduced cytochrome c or ribo-nuclease A (RNase) have so far failed. These platinum complexes are known to bind to the sulphur atoms of exposed methionines (residues 65 and 29 of Cyt c and RNase respectively) as shown by our previous H NMR studies on RNase (28) and those of Boswell et al on Cyt c (29) and x-ray crystallography. We assume therefore that the resonances are broadened beyond detection via chemical shift anisotropy relaxation. The restriction of Pt mobility on the protein will lead to a large increase in (see CSA equation above). The anisotropy term would also be expected to increase. Scalar coupling to N will also contribute to the increase in linewidths if nitrogen binding sites are involved. 

We describe here the results of C NMR linewidth, Ti, and NOE measurements on the mobile domains of solid Hytrel. For the rigid domains we present the results of cross polarization rate measurements and reconstructions of the chemical shift anisotropies (14). Taken together, these results provide us with new insights into the domain structure and molecular dynamics of a segmented copolymer which does not have a discrete domain structure. 

In other studies the mechanism of cross-linking in irradiated copolymers of tetrafluoroethylene and perfluoromethylvinylether has been studied by Forsythe e/o/. " This material is rubbery at room temperature, and hence sufficient molecular mobility is available to permit radical-radical recombination reactions. Thus the material undergoes both chain scission and cross-linking. The authors were able to quantify the yields of new products using F MAS NMR at moderate spinning speeds, since the chemical shift anisotropy and homonuclcar dipole-dipole couplings of the fluorine nuclei are partially averaged by rapid molecular motion in the rubbery state. 

MAS NMR spectroscopy of the spin nuclei H, Si and P has been particularly useful in studies of microporous solids but many other nuclei of interest in the area of microporous solids are quadrupolar, and indeed 74% of NMR-active nuclei have a spin greater than Among these, is the only nucleus of major chemical interest with an integer spin (I = 1), and is of particular relevance to porous solids for studies of mobility. By contrast, many quadrupolar nuclei with non-integral spin are important, including Na, A1 and Ga. The central (mi= + 5 -5) transition of quadrupolar nuclei is to first order unaffected by quadrupolar coupling, and since dipolar coupling and chemical shift anisotropy are removed by MAS, reasonable... 

The P chemical shift for neat DMMP (Fig. 6a) is 33 ppm. When DMMP is adsorbed in nanociystalline NaY, the peak shifts slightly downfield to 34 ppm and spinning sidebands (marked with asterisks) appear in the NMR spectrum (Fig. 6b). The spinning sidebands indicate appreciable chemical shift anisotropy and a decrease in mobility suggesting that DMMP strongly adsorbs on the nanociystalline NaY surface. When water is coadsoibed with DMMP, the peak shifts further downfield to 37 ppm and is narrower although spinning sidebands are still observed in the NMR spectrum (Fig. 6c). 

Since the time dependence of variations in magnetization, dipolar or quad-rupolar coupling, and chemical-shift anisotropy are inherent parts of the NMR phenomenon, it is a natural tool for studying molecular motions. These relationships are usually well understood, and NMR data can therefore be used to test dynamic models. Conformations, free volume, diffusion, and mechanical properties have all been correlated with NMR observations. This section will first review mobile systems (solution, melts, and elastomers), and then discuss solids. 

Solid-state C-NMR spectroscopy has proven to be also a valuable tool for studying the properties of buckminsterfullerene. In Fig. 4.33 a series of spectra of a solid sample of C o at different temperatures is reproduced. The single, relatively narrow signal observed at room temperature indicates that under these conditions the molecules reorient rapidly and isotropically in the solid. However, as the temperature goes down, a broad pattern characteristic for the chemical shift anisotropy of powder samples is observed. The evolution of the spectrum described in Fig. 4.33, corresponding to the transformation of a situation of rapid molecular rotation into one characterized by static molecules, points to a phase equilibrium between a mobile phase called the rotator phase ... 

---