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Substituent induced chemical shift additivity

The last section is a compilation of useful empirical additivity rules for 13C chemical-shift prediction, regardless of underlying transmission mechanisms. Peculiarities of less common substituents are cited, and, finally, substituent-induced 13C signal shifts in various cyclic compounds are discussed, with special emphasis on their use in conformational analysis I do not, however, claim completeness for this latter discussion. [Pg.222]

Tables 5.2 and 5.3 give characteristic shifts for nuclei in some representative organic compounds. Table 5.4 gives characteristic chemical shifts for protons in common alkyl derivatives. Table 5.5 gives characteristic chemical shifts for the olefinic protons in common substituted alkenes. To a first approximation, the shifts induced by substituents attached an alkene are additive. So, for example, an olefinic proton which is trans to a -CN group and has a geminal alkyl group will have a chemical shift of approximately 6.25 ppm [5.25 + 0.55(tra .s-CN) + 0.45(gew-alkyl)]. Tables 5.2 and 5.3 give characteristic shifts for nuclei in some representative organic compounds. Table 5.4 gives characteristic chemical shifts for protons in common alkyl derivatives. Table 5.5 gives characteristic chemical shifts for the olefinic protons in common substituted alkenes. To a first approximation, the shifts induced by substituents attached an alkene are additive. So, for example, an olefinic proton which is trans to a -CN group and has a geminal alkyl group will have a chemical shift of approximately 6.25 ppm [5.25 + 0.55(tra .s-CN) + 0.45(gew-alkyl)].
A linear correlation between 13C chemical shifts and local n electron densities has been reported for monocyclic (4n + 2) n electron systems such as benzene and nonbenzenoid aromatic ions [76] (Section 3.1.3, Fig. 3.2). In contrast to theoretical predictions (86.7 ppm per n electron [75]), the experimental slope is 160 ppm per it electron (Fig. 3.2), so that additional parameters such as o electron density and bond order have to be taken into account [381]. Another semiempirical approach based on perturbational MO theory predicts alkyl-induced 13C chemical shifts in aromatic hydrocarbons by means of a two-parameter equation parameters are the atom-atom polarizability nijt obtained from HMO calculations, and an empirically determined substituent constant [382]. [Pg.254]

The nmr spectra of bridgehead substituted adamantanes are generally readily analyzed since, for the most part, only minor spin-spin coupling is observed in these substrates. The chemical shifts induced by various bridgehead substituents are found to be remarkably additive 150T Knowing the chemical shifts pro-... [Pg.42]

Pevzner et al. widely used NMR spectroscopy in their structural investigations of diverse 1,2,4-triazole nitro derivatives [566-581], In the series of 5-substituted l-methyl-3-nitro-1,2,4-triazoles the correlation between the A-methyl group proton chemical shifts induced by 5-substituents (A8) and the substituent Hammett constants has been found to divide into two branches [577], This nonordinal event is explained by impossibility of any additional contribution to the shielding of substituents having two and more lone electron pairs [577],... [Pg.227]

There is also a correlation of C NMR chemical shifts of the a- and p-carbons of di- and tri-sub-stituted a,p-unsaturated acids, represented by the empirical relation in equations (2a) and (2b), which relies on the additive shielding increments Another method used for the determination of double bond configuration makes use of effects induced by benzene or other aromatic solvents on the chemical shifts of double bond substituents preferential shielding of protons or alkyl groups trans to the polar groups is usually observed. An example of the use of this method is the determination of the stereochemistry of the double bond in (Z)- and ( -geranic acids (6 and 7 Scheme 6). ... [Pg.346]


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See also in sourсe #XX -- [ Pg.269 ]




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