Isotropical chemical shift

Predict the isotropic chemical shift for carbon in benzene with respect to TMS. 

At low rotation rates, less than the chemical shifts anisotropy, however, the powder spectra contained disturbing side bands dispersed among the isotropic chemical shifts. In order to discriminate between sidebands and isotropic resonances two spectra obtained at different spinning speeds were multiplied together or the differentiation was made by visual inspection. 

In addition to sample rotation, a particular solid state NMR experiment is further characterized by the pulse sequence used. As in solution NMR, a multitude of such sequences exist for solids many exploit through-space dipolar couplings for either signal enhancement, spectral assignment, interauclear distance determination or full correlation of the spectra of different nuclei. The most commonly applied solid state NMR experiments are concerned with the measurement of spectra in which intensities relate to the numbers of spins in different environments and the resonance frequencies are dominated by isotropic chemical shifts, much like NMR spectra of solutions. Even so, there is considerable room for useful elaboration the observed signal may be obtained by direct excitation, cross polarization from other nuclei or other means, and irradiation may be applied during observation or in echo periods prior to... 

The probe molecules of greatest historical interest in catalysis are the Hammett indicators [13]. The difficulty of making reliable visual or spectrophotometric observations of the state of protonation of these species on solids is well known. We have recently carried out the first NMR studies of Hanunett indicators on solid acids [ 14]. This was also the occasion of the first detailed collaboration between the authors of this article, and theoretical methods proved to strongly compliment the NMR experiments. The Hanunett story is told after first reviewing the application of theoretical chemistry to such problems. Central to the application of any physical method in chemistry is the process of modeling the relationship between the observables and molecular structure. However often one does this, it is rarely an exact process. One can rationalize almost any trend in isotropic chemical shift as a function of some variation in molecular structure - after the fact, but the quantitative prediction of such trends in advance defies intuition in most nontrivial cases. Even though the NMR spectrum is a function... 

The large downfield CSA for disilenes, and indeed the large isotropic chemical shift, is caused mainly by the great deshielding of one component of the tensor, o-,. Tossell and Lazzaretti propose that this deshielding results from a low-energy electronic transition between a o--bonding orbital in the molecular plane and the Si=Si 7r -orbital.45... 

Thus the chemical shift in the solid state has three components in the directions of 3 orthogonal axes. MAS allows us to obtain the isotropic chemical shift, the quantity which we measure in solution and which thus interests us. 

Fig. 53 74.63 MHz CP-MAS spectrum of C(SnMe3)4, isotropic chemical shift 48.2 ppm, 2J(119Sn-119Sn) 328 Hz (the coupling visible is that between a tin-119 and a tin-117 nucleus). Both the isotropic chemical shift and the two-bond tin-tin coupling correspond to the solution values... 

The estimated distribution of A1 atoms is shown in Figure 2. Both the siting as well as therelative concentration of A1 in the individual T sites significantly vary. Our QM-Pot calculations of ZSM-5 structures (PI symmetry) containing one A1 atom per unit cell resulted in the 24 structures corresponding to A1 substitution into the 24 distinguishable T sites of the monoclinic ZSM-5 and yielded 24 values of shieldings which were converted into 24 values of isotropic chemical shift. 

Figure 3. Comparison of observed and calculated isotropic chemical shifts of the eleven ZSM-5 samples.

We have demonstrated that a combined experimental (27A1 3Q MAS NMR) and theoretical (QM-Pot employing the bare framework model) approach represents a powerful tool for the determination of the local geometry of framework A104 tetrahedra, the prediction of27A1 isotropic chemical shifts in hydrated silicon rich zeolites, and the identification of A1 siting in the framework of silicon-rich zeolites. Experimental evidence is provided for the occupation of at least 10 out of 24 distinguishable framework T sites by A1 atoms in silicon-rich ZSM-5. The conclusion is reached that the A1 distribution over the framework T sites is neither random nor controlled by a simple rule, but depends on the conditions of the zeolite synthesis. 

The REDOR experiment (Fig. la), introduced by Gullion and Schaefer in 1989 [22], invokes a strong n pulse for one of the spin species (e.g., the I spin in an I-S spin system) in the middle of each rotor period xr - in addition to one n pulse at each rotor echo on the 5-spin channel to refocus isotropic chemical shift effects and... 

The calculation of the effective Hamiltonian is greatly simplified by only considering the isotropic chemical shift difference, Aoj1so = cof° — a> f leading to... 

As demonstrated by Griffin, Levitt, and coworkers in the late 1980s [21, 93], it is also possible to recouple homonuclear dipolar couplings through interference between isotropic chemical shifts and the rotor revolution. This phenomenon, called rotational resonance, occurs when the spinning frequency is adjusted to a submultiple of the isotropic chemical shift difference, i.e., ncor = ct> so o) °. To understand this experiment, the dipolar coupling Hamiltonian in (10) is transformed... 

Isotropic Chemical Shift and Second-Order Quadrupolar Induced Shift... 

Let us calculate the frequencies of transitions between Zeeman eigenstates s) and r), assuming that the nuclei are only subjected to an isotropic chemical shift and the first- and second-order quadrupolar interaction. As seen in Sect. 2.1, the Hamiltonian that governs the spin system in the frame of the Zeeman interaction (the rotating frame) is... 

In the fast-spinning case Tics = resonance offset or the isotropic chemical shift and the quadrupolar terms are given by (16) and (18). Thus, we obtain... 

The isotropic chemical shift, the trace of the chemical shift tensor, is one of the basic NMR parameters often measured for both spin-1/2 and quadrupolar nuclei. The CSA can also be measured in non-cubic environments, such as the n3Cd nuclei experience in the chalcopyrite structure of crystalline CdGeAs2 [141] or CdGeP2 [142], and the 31P nuclei in the latter compound [142], Although the isotropic chemical shift can be measured from the NMR spectrum of a static powder because the CSA is zero in many cases because of cubic symmetry of the lattice, improved resolution is obtained by using MAS to remove dipolar couplings. Two particular areas where the isotropic chemical shifts have proven very informative will now be discussed, semiconductor alloys and semiconductor polytypes. 

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