The anisotropic 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). 

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. 

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. 

Figure 1 Calculated ring current effects of 1,2-dloxln, 1,2-oxathlln, and 1,2-dithiin (in comparison with the ring current effects of cyclobuta-1,3-dlene, benzene and the anisotropic effect of buta-1,3-dlene) shielding isochemical shielding surface (ICSS) of -0.1 ppm, gray, and deshielding ICSS of 0.1 ppm, dark gray.

These hybridisation variations are caused by anisotropy within the chemical bonds. This is due to the non-homogeneous electronic distribution around bonded atoms to which can be added the effects of small magnetic fields induced by the movement of electrons (Fig. 9.12). Thus, protons on ethylene are deshielded because they are located in an electron-poor plane. Inversely, protons on acetylene that are located in the C-C bond axis are shielded because they are in an electron-rich environment. Signals related to aromatic protons are strongly shifted towards lower fields because, as well as the anisotropic effect, a local field produced by the movement of the aromatic electrons or the ring current is superimposed on the principal field (Fig. 9.12). 

Carbon-13 shifts of alkynes (Table 4.13) [246-250] are found between 60 and 95 ppm. To conclude, alkyne carbons are shielded relative to olefinic but deshielded relative to alkane carbons, also paralleling the behavior of protons in proton NMR. Shielding relative to alkenes is attributed to the higher electronic excitation energy of alkynes which decreases the paramagnetic term according to eq. (3.4), and to the anisotropic effect of the triple bond. An increment system can be used to predict carbon shieldings in alkynes... 

The H-NMR spectra of the isomeric 2-oxo-2H- and 4-oxo-4//-pyrido-[l,2-fl]pyrimidines exhibit some characteristic differences. In the spectra of 4-oxo-4//-pyrido[l,2-a] pyrimidines, which are unsubstituted at position 6, the anisotropic effect of the adjacent carbonyl group causes the signal of the H-6 to be shifted downfield by about 1 ppm as compared with the corresponding signal for the 2-oxo-2H isomer.2 19,24 86... 

The 2,2,6,6-tetramethylpiperidinoxyl radical (TEMPO) was first prepared in 1960 by Lebedev and Kazarnovskii by oxidation of its piperidine precursor.18 The steric hindrance of the NO bond in TEMPO makes it a highly stable radical species, resistant to air and moisture. Paramagnetic TEMPO radicals can be employed as powerful spin probes for elucidating the structure and dynamics of both synthetic and biopolymers (e.g., proteins and DNA) by ESR spectroscopy.19 Unlike solid-phase 1H-NMR where magic angle spinning is required in order to reduce the anisotropic effects in the solid-phase environment, solid-phase ESR spectroscopy can be conducted without specialized equipment. Thus, we conducted comparative ESR studies of various polymers with persistent radical labels, and we also determined rotational correlation times as a function of... 

Figure 25 displays the anisotropic effective oxygen diffusivity variations with liquid water saturation in the GDL based on the evaluated pore blockage correlations. Furthermore, the impact of GDL compression on the pore blockage effect was also investigated.67... 

Fig. 3. The proton chemical shifts of the heterobenzenes vs. R 3, where R is the interatomic distance between the group 5 atom and H , He or Hr The dashed lines indicate the estimated magnitude of the ortho-inductive effects. The slope of the lines indicates the magnitudes of the anisotropic effects of the group 5 atoms, while the intercept indicates the chemical shift value expected in the absence of this anisotropic effect...

In order to remove the anisotropic effects of quadrupolar interactions which can obscure MAS spectra other techniques such as Double Rotation (DOR) [18, 19], Dynamic Angle Spinning (DAS) [18] or Multiple Quantum Magic Angle Spinning (MQMAS) have to be employed. Of these the most simple to apply practically is MQMAS, which can be conducted on a standard MAS-equipped NMR spectrometer. A number of texts and review papers discuss MQMAS in significant technical detail [20, 21]. 

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