Chemical-shift interaction

From the NMR data of the polymers and low-molecular models, it was inferred that the central C—H carbons in the aliphatic chain in polymer A undergo motions which do not involve the OCH2 carbons to a great extent. At ambiet temperatures, the chemical shift anisotropy of the 0(CH2)4 carbons of polymer A are partially averaged by molecular motion and move between lattice positions at a rate which is fast compared to the methylene chemical shift interaction. 

The study of local or long-range ordering in semiconductor alloys based upon the effects on NQCCs has been carried out in a number of cases. These studies are analogous to the study of ordering in In Ga P by means of the chemical shift interaction, as described in Sect. 3.3.1. 

The use of solid state NMR for the investigation of polymorphism is easily understood based on the following model. If a compound exists in two, true polymorphic forms, labeled as A and B, each crystalline form is conformationally different. This means for instance, that a carbon nucleus in form A may be situated in a slightly different molecular geometry compared with the same carbon nucleus in form B. Although the connectivity of the carbon nucleus is the same in each form, the local environment may be different. Since the local environment may be different, this leads to a different chemical shift interaction for each carbon, and ultimately, a different isotropic chemical shift for the same carbon atom in the two different polymorphic forms. If one is able to obtain pure material for the two forms, analysis and spectral assignment of the solid state NMR spectra of the two forms can lead to the origin of the conformational differences in the two polymorphs. Solid state NMR is thus an important tool in conjunction with thermal analysis, optical microscopy, infrared (IR) spectroscopy, and powder... 

The electrons modify the magnetic field experienced by the nucleus. Chemical shift is caused by simultaneous interactions of a nucleus with surrounding electrons and of the electrons with the static magnetic field B0. The latter induces, via electronic polarization and circulation, a secondary local magnetic field which opposes B0 and therefore shields the nucleus under observation. Considering the nature of distribution of electrons in molecules, particularly in double bonds, it is apparent that this shielding will be spatially anisotropic. This effect is known as chemical shift anisotropy. The chemical shift interaction is described by the Hamiltonian... 

It has been shown before that the removal of aluminum from the framework leads to very marked narrowing of NMR signals. When no A1 is present, the Si(4Si) lines are very narrow indeed ( 0.7 ppm). Fyfe et al. (84-88) studied the effect of dealumination on linewidths of Si(4 Si) signals in several zeolites. They found that substantial line narrowing occurs at Si/Al > 100, which indicates that the effect must be long range in nature. They suggest (86) that it is caused by a chemical shift interaction due to distribution of Al in the second-nearest and further coordination shells of silicon. 

Butene trans-But-2-ene (Ge, Al)-X T, chemical shifts, interactions with cations (307)... 

Fig. 83. Pulse sequence for the 2D SLF spectrum of 1 JC- H interactions (414). During evolution period, r, mainly heteronuclear spin interactions are effective, whereas chemical shift interactions govern the time evolution during the detection period, r2 (see text).

The isotropic chemical shift is the average value of the diagonal elements of the chemical shift tensor. Advances in solid state NMR spectroscopy allow one to determine the orientation dependence, or anisotropy, of the chemical shift interaction. It is now possible to determine the principal elements of a chemical shift powder pattern conveniently, and the orientation of the principal axes with more effort. Hence, instead of settling for just the average value of the chemical shift powder pattern, one can now aim for values of the three principal elements and the corresponding orientations in a molecular axis system. 

Solid State 170 NMR Spectral Analysis. The 170 nucleus has a 5/2 spin and so has a quadrupolar moment. This means that static and MAS 170 NMR spectrum contains information about quadrupolar interactions and the chemical shift. In order to obtain the quadrupolar coupling constant and chemical shift separately, we have carried out spectral analysis with theoretical calculation taking into account quadrupolar interactions and chemical shift interactions. 

There have been a number of publications on correlation or separation experiments involving quadrupolar nuclei. Using switching angle spinning (SAS), the correlation of quadrupolar and chemical shift interactions between the two spinning axes can be established to extract the quadrupolar and chemical shift tensors as well as their relative orientations.199... 

This technique was extended by Schmidt-Rohr et al.251 and was given the acronym DECODER (direction exchange with correlation for orientation-distribution evaluation and reconstruction). The pulse sequence of the 2D version of this method, which is shown in Fig. 16(a), is basically a typical 2D exchange experiment except that the sample is reoriented during the mixing time. Consequently, the correlation of, for example, the chemical shift interaction at two different sample orientations is obtained. 

Figure 38. (left) Solid-echo 2H NMR spectra of glycerol-/ (7 = 189 K) [305]. A collapse of the solid-state spectrum is observed upon heating the corresponding time constants of the a-process are indicated, (right) Hahn-echo 31P NMR spectra of w-tricresyl phosphate (m-TCP, Tg = 210K) determined by the anisotropic chemical shift interaction [324]. 

Mansfield and Ware76 of extending the time domain signal of dipolar coupled systems by applying a cyclic train of 90° pulses. An MP sequence typically consists of a train of several hundred RF pulses comprising repeated sets of n pulses,2 with 4< <48. If the sequence is applied at a rate sufficiently rapid compared with the value of the linewidth to be narrowed, it will average to zero dipolar interactions within the sample, while the chemical-shift interaction is either scaled or eliminated, depending on the type of MP sequence used. 

In multidimensional NMR studies of organic compounds, 2H, 13C and 31P are suitable probe nuclei.3,4,6 For these nuclei, the time evolution of the spin system is simple due to 7 1 and the strengths of the quadrupolar or chemical shift interactions exceed the dipole-dipole couplings so that single-particle correlation functions can be measured. On the other hand, the situation is less favorable for applications on solid-ion conductors. Here, the nuclei associated with the mobile ions often exhibit I> 1 and, hence, a complicated evolution of the spin system requires elaborate pulse sequences.197 199 Further, strong dipolar interactions often hamper straightforward analysis of the data. Nevertheless, it was shown that 6Li, 7Li and 9Be are useful to characterize ion dynamics in crystalline ion conductors by means of 2D NMR in frequency and time domain.200 204 For example, small translational diffusion coefficients D 1 O-20 m2/s became accessible in 7Li NMR stimulated-echo studies.201... 

The chemical shift interaction results in a shift in the basic resonance frequency of the nucleus being examined and arises from the concurrent interaction of... 

Following an rf pulse, because the tensor orientations of each crystallite are different, the resonant frequency for each crystallite is different and the magnetisation rapidly dephases. This can be envisaged pictorially from the chemical shift interaction. In the static powder pattern the frequency axis could be read as an orientation axis. Then in Figure 2.12 the two sets of spins starting off at A and B have different initial precession rates. The azimuthal phase angle picked up by each of these orientations after a time t is... 

Figure 2.12. Schematic variation of the frequency of two spin packets under a chemical shift interaction showing that the frequency varies during the MAS period (Tr) and that the same range of frequencies are experienced by the two spin packets.

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