Bonds 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). 

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. 

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... 

Due to cooperating anisotropic effects, shieldings of central sp carbon atoms increase with the number of conjugated triple bonds in polyalkynes (Table 4.14). 

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... 

In contrast with the striking anisotropic effects of circulating v electrons, the a electrons of a C—C bond produce a small effect. For example, the axis of the C—C bond in cyclohexane is the axis of the deshielding cone (Figure 3.24). The observation that an equatorial proton is consistently found further to the left by 0.1-0.7 ppm than the axial proton on the same carbon atom in a rigid six-membered ring can thus be rationalized. The axial and equatorial protons on Q are oriented similarly with respect to C,—C2 and C,—C6, but the equatorial proton is within the deshielding cone of the C2—C3 bond (and C.-Q). 

Morphine alkaloids have been used as model compounds in a study of double bond and nitrogen and nonbonding electron anisotropic effects/391,394 The ABC system of protons at 9, 10a, and 10/ of several alkaloids of the morphine group has been the subject of theoretical analysis/395 ... 

If molecules contain protons that are sterically oriented so that they are close to double or triple bonded groups, there will generally be a diamagnetic anisotropic effect of some kind. 

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