Anisotropic effect

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). 

Multiple bonds are revealed clearly by anisotropic effects. Textbook examples include alkynes, shielded along the C=C triple bond, and alkenes and carbonyl compounds, where the nuclei are deshielded in the plane of the C=C and C=0 double bonds, respectively One criterion for distinguishing methyl groups attached to the double bond of pulegone (31), for example, is the carbonyl anisotropic effect. 

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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. 

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. 

If the Ln3+ centre is a Kramers ion, the spectra can be interpreted in terms of a doublet with largely anisotropic effective -values. If one neglects the admixture of higher lying/ multiplets and considers an axial symmetry, the effective g values will be... 

With the structure determined, a detailed analysis of the 400-MHz H-NMR spectrum was performed in comparison with other ervafolines (Table XI) (214). Characteristic were the singlet at 3.86 ppm for H-3 and the multiplet at 5.64 ppm for aromatic H-12. The unusual shift of the latter proton is due to the anisotropic effect of the neighboring aromatic ring in the lower part (part B) of the dimer. 

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. 

For pores of extremely small diameters, in the order of a few nm, the direction of individual pores is totally random. On the other hand, large pores tend to have less anisotropic effect and grow more dominantly in the direction of carrier supply, that is, perpendicular to the surface. The macro pores formed on p-Si generally have smooth walls and an orientation toward the source of holes that is perpendicular to the surface, even on (110) and (111) samples.34,39... 

The rate of an electrode reaction is a function of three principle types of species charge carriers on the surface, active surface atoms and reactant species in the solution as illustrated in Figure 23. That is, r cc [h] [Siactive] [A]. Carrier concentration and reactant concentration do not, in general, depend on surface orientation while active surface atoms may be a function of surface orientation. Anisotropic effect occurs when the rate determining step depends on the active surface atoms that vary with crystal orientation of the surface. On the other hand, reactions are isotropic when the concentration of active surface atoms is not a function of surface orientation or when the rate determining step does not involve active surface atoms. 

For moderately doped substrates, when the surface is free of oxide the change of potential is mostly dropped in the space charge layer and in the Helmholtz double layer. The reactions are very sensitive to geometric factors. The reaction that is kinetically limited by the processes in the space charge layer is sensitive to radius of curvature, while that limited by the processes in the Helmholtz layer is sensitive to the orientation of the surface. Depending on the relative effect of each layer the curvature effect versus anisotropic effect can vary. 

Orientation, branching and straightness of pores are related to similar factors, namely source of holes and anisotropic effect. Orientation of pores can be explained based on the current difference between (100) and (HI) orientations, Ai = i - i, and the difference between current densities on the pore bottoms of main pores and on side pores, ihoie, mam and ihoie> side, as shown in Figure 27. ihoie> Slde is smaller than... 

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