Shift , chemical anisotropic effects

Figure 1 Solid-state NMR powder patterns, dominated by chemical shift anisotropy effects (a) spherically symmetric chemical shift tensor, (b) axially symmetric chemical shift tensor, (c) asymmetric chemical shift tensor. Top traces theoretical powder patterns bottom traces powder patterns broadened by other anisotropic interactions or chemical shift distribution effects.

An E-Z discrimination between isomeric oxaziridines (27) was made by NMR data (69JCS(C)2650). The methyl groups of the isopropyl side chains in the compounds (27) are nonequivalent due to the neighboring carbon and nitrogen centres of asymmetry and possibly due to restricted rotation around the exocyclic C—N bond in the case of the Z isomer. The chemical shift of a methyl group in (Z)-(27) appears at extraordinarily high field, an effect probably due to the anisotropic effect of the p-nitrophenyl group in the isomer believed to be Z. 

The chemical shift of a nucleus depends in part on its spatial position in relation to a bond or a bonding system. The knowledge of such anisotropic effects is useful in structure elucidation. An example of the anisotropic effect would be the fact that axial nuclei in cyclohexane almost always show smaller H shifts than equatorial nuclei on the same C atom (illustrated in the solutions to problems 37, 47, 48, 50 and 51). The y-effect also contributes to the corresponding behaviour of C nuclei (see Section 2.3.4). 

Diorganotin(IV) complexes 109 were characterized by NMR spectroscopy (96MI4). The downfield chemical shift of 6-H in 2-fluoroalkyl-4//-pyrido[l,2-n]pyrimidin-4-ones 111 is attributed to the anisotropic effect of the 4-carbonyl group (97JCS(P1)981). 

Ring current (anisotropic) effects do not play a significant role in fluorine NMR. Therefore, fluorine substituents on a benzene ring absorb in the general region of fluoroalkenes, with fluorobenzene and 1-fluoronaphthalene having chemical shifts of -113.5 and -123.9 ppm, respectively. The fluorine NMR of fluorobenzene is shown in Fig. 3.14. 

Structure 6.8 demonstrates a most extreme example of anisotropy. In this unusual metacyclophane, the predicted chemical shift (Table 5.8) of the methine proton that is suspended above the aromatic ring would be 1.9 ppm. In fact, the observed shift is -4 ppm, i.e., 4 ppm above TMS The discrepancy between these values is all down to the anisotropic effect of the benzene ring and the fact that the proton in question is held very close to the delocalised p electrons of the pi cloud. 

ACD/Labs have an extensive database which uses this approach. This approach works well except for anisotropic groups. Unlike carbon prediction this can have a massive effect on the chemical shift values and so can give rise to big errors in prediction, even for structural fragments that are well represented in... 

In the simplest setup, the two strong field components may be set identical to Ci = Cs = C. The relatively large CIX or CSX term averages isotropic and anisotropic chemical shift effects as well as the heteronuclear dipolar coupling interaction between 15N or 13C and H. The difference of - or the sum of - the B coefficients selects the form of the recoupled heteronuclear dipole-dipole coupling interaction, as expressed in terms of the effective Hamiltonian in the interaction frame of the rf irradiation... 

Anisotropic effect (Also called space effect). The 8 value (chemical shift) in each case can be justified by explaining the manner in which n electrons circulate under the influence of the applied. 

Carbon Chemical Shifts. In general, because carbon chemical shift values are dominated by the paramagnetic term, they are sensitive to a variety of factors such as hybridization, inductive, electric field, steric , resonance, and neighboring anisotropic effects. In many instances trends can be found for a series of compounds, but interpretation of the results is difficult. Therefore, it can be more enlightening to simply look for where changes in chemical shift do and do not occur and correlate them to better characterized data. 

The NMR observables of a substance as a liquid, solution, or in the gas phase of paramount importance to the inorganic chemist are chemical shifts and nuclear spin-spin coupling constants. In the solid state, however, these effects generally are dwarfed by anisotropic dipolar interactions so... 

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