Deshielding, paramagnetic

This indicates that the deviations are due to systematic errors, for example deficiencies of the applied methods and basis sets. DFT-based methods, such as GIAO/DFT calculations are known to overestimate paramagnetic contributions to the chemical shielding. This results, for critical cases with small HOMO/LUMO separations, in overly deshielded competed chemical shifts. Notorious examples for these deficiencies are 29Si or 13C NMR chemical shift computations of silylenes, silylium ions or dienyl cation .(5/-54) Taking into account the deficiencies of the applied method, and bearing in mind very reasonable correlations shown in Figures 4 and 5, the computational results do support the structural characterization of the synthesized vinyl cations. 

Shabab (45) attributed the a-SCS to a subtle balance of both the paramagnetic and diamagnetic (40-42) shielding contributions in which the latter increases more rapidly than the former with increasing atomic number of X. Thus, the net observed effect becomes less deshielding as the atomic number of X increases (45). 

For nuclei other than hydrogen, such as 13C, the paramagnetic shielding term [Pg.110]

The pseudocontact term Afipo,ar is due to the valence electrons of the shift reagent SR they cause an additional intramolecular field in the adduct S + SR, shielding or deshielding the nucleus i in the molecule S. The Fermi contact term Afon ac accounts for interaction between the nucleus i and the field of the unpaired electrons of the paramagnetic additive SR which may be delocalized within the adduct S + SR [103]. 

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... 

N-Quaternization and the formation of N-oxides of alkaloids cause strong paramagnetic deshielding effects for the carbon atoms neighboring the nitrogen atom however the carbons located at /(-position usually become more shielded [600, 606, 634, 643],... 

The reactivity of the carbon atom raises questions about its chemical nature should it be regarded as cationic, neutral (carbenoid perhaps) or anionic, as the name carbidocarbonyl has long implied. The available, 3C NMR data, in which the carbide resonances are significantly to low field, may be interpreted in terms of a deshielded positively charged carbon atom, and such a suggestion has been made (80). However, the cause of the shifts observed for cluster bound carbon atoms is not clear and may reflect the predominance of paramagnetic contributions to the chemical shift rather than simply deshieiding at a cationic center. 

The MO calculations on model disilenes indicate that the deshielding results from a paramagnetic contribution along the in-plane axis perpendicular to the Si—Si vector. Implications of the NMR data and the theoretical computations for Si=Si bonding are discussed. 

Certain protons of fused troponoids show remarkable low-field shifts (Table XII, Scheme 75 for comparison, see Tables IX and X). These protons are abnormally deshielded by the paramagnetic anisotropy effect of peri-positioned or other neighboring groups, especially the tropone carbonyl groups. Similar low-field shifts are caused by adjacent nitro or halogen substituents [e.g., 74YZ1445 80JCS(P1)2081],... 

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