Substituent-induced chemical shifts

Table 15 contains the C chemical shifts of some selected indazoles. The major difference between indazoles and isoindazoles lies in the chemical shifts of carbons C-3 and C-7a. The substituent chemical shifts (SCS) induced by the substituent in position 3 have been discussed using an empirical model (770MR(9)716). The model that gives the best results, AS = OS + + cSl and 3 are the Swain-Lupton parameters and 5 is the Schaefer... 

When the donor group is attached at a /3-pyrrole position rather than the meso position, the properties of the assemblies change. Knapp (57) synthesized a series of /3-pyrrole 2-pyridyl substituted zinc porphyrins (49, Fig. 16) and showed that dimerization takes place provided the meso-substituents are not too bulky H NMR and VPO measurements showed that the porphyrin, which bears heptyl substituents, dimerizes in solution, whereas the phenyl analog does not. The complexation-induced changes in chemical shift suggest a stacked structure for the dimer. 

One other, perhaps even more dramatic and common example concerns compounds like 2 and 4 hydroxy- and amino-pyridines. These compounds exhibit tautomeric behaviour and tend to exist in solution as the corresponding pyridone and imine. This reduces the familiar pyridine-like properties of the ring system, accentuating the effects of these substituents (in terms of induced chemical shifts) and at the same time, radically increasing the expected couplings 2 -3 couplings. 

Sc, carbon chemical shift, referred to tetramethylsilane (8 = 0) (cf. Sect. I) SCS, substituent-induced chemical shift, or substituent effect difference between S s of a given carbon atom in a monosubstituted and the respective unsubstituted parent molecule (cf. Sect. Ill) NAE, nonadditivity effect nonadditivity of individual SCSs in disubstituted molecules (cf. Sect. IV) ICS, intramolecular-interaction chemical shift = NAE (cf. Sect. IV) A, polarization effect difference in S s of sp2 carbon atoms in a double bond (cf. Sect. IV-C) LEF, linear electric field (cf. Sect. II-B-3) SEF, square electric field (cf. Sect. II-B-3). 

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. 

Replacement of a hydrogen atom within an organic molecule, for example an alkane, by a substituent X changes the electronic environments of directly bonded and of more remote carbon nuclei. Thereby l3C NMR signals are shifted either upheld or downfield the difference between the chemical shifts 8 of a certain carbon atom in the substituted and the unsubstituted parent compound is called the substituent effect. For this term the abbreviation SCS (substituent-induced chemical shift) has generally been adopted in the literature and will also be used here. The SCS is given by the equation... 

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)].

The chemical shift effects of nitro and amino substituents are the largest normally encountered. The mildly electron donating methyl group induces far more modest shifts, as seen with 2- and 3-methylthiophene (113 and 114). 

Correlations between substituent-induced chemical shift differences and reactivity parameters have been examined. Good linear correlations have been obtained using the Swain-Lupton two-parameter equation ... 

The 13C chemical shifts of benzenoid carbons largely depend on the mesomeric interaction between substituent und benzene ring. Electron releasing substituents (e.g. — NH2, — OH) will increase the electron density at the o and p carbons relative to benzene (128.5 ppm), while slight electron deficiencies will be induced by electron withdrawing groups (e.g. — N02, —CN). 

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]. 

Up to this time there has been no report of the experimental determination of the structure of the parent homotropenylium ion. The three simplest systems that have been studied are 18, 19 and the iron complex 20. Cations 18 and 19 each have an oxygen-containing electron-donor substituent and, as such, appear to have smaller induced ring currents than the parent ion. In fact 18 and 19 have almost identical chemical shift differences (A<5 = 3.10 ppm) between the two C(8) protons. In the case of 20, A<5 is very small and it was considered to be a non-cyclically delocalized model for the bicyclo[5.l.OJheptadienyl cation69. 

H and, 3C NMR spectra of monoimines have been reported and are shown in Table 3. These data fit the Hammett relationship and good correlations are found between various o constants and, 3C chemical shifts at positions remote from the substituent.1413C chemical shifts of imines derived from BA and variously substituted benzylamines are affected by substituents through up to 11 bonds.15 The influence of ZjE isomerism (Figure 3) and the effect of substituents on NMR spectra of some unsaturated imines have been studied.16 The photochemically induced isomerization of several substituted Schiff bases derived from BA and arylamines and their chemical shifts, as well... 

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