Substituent chemical shifts , acceptor

Representative chemical shifts from the large amount of available data on isothiazoles are included in Table 4. The chemical shifts of the ring hydrogens depend on electron density, ring currents and substituent anisotropies, and substituent effects can usually be predicted, at least qualitatively, by comparison with other aromatic systems. The resonance of H(5) is usually at a lower field than that of H(3) but in some cases this order is reversed. As is discussed later (Section 4.17.3.4) the chemical shift of H(5) is more sensitive to substitution in the 4-position than is that of H(3), and it is also worth noting that the resonance of H(5) is shifted downfield (typically 0.5 p.p.m.) when DMSO is used as solvent, a reflection of the ability of this hydrogen atom to interact with proton acceptors. This matter is discussed again in Section 4.17.3.7. 

A plot of the fluorine-19 chemical shift for para-substituted fluorobenzenes versus the carbon-13 chemical shift for the corresponding substituted benzenes also gives a good bilinear relationship. The smaller slope for the n acceptor substituents is in agreement with canonicals of the form ... 

NMR spectroscopy is also a valuable probe for hexacarbonylalkyne-dicobalt complexes, and spectroscopic data for 34 representative [Co2 (iu-alkyne)(CO)6] complexes have been tabulated.44 Correlation of the 13C NMR chemical shifts of the coordinated sp carbon atoms with those of the corresponding free alkynes reveals that the interaction between the substituent groups and the C2Co2(CO)6 moiety is very sensitive not only to the donor/acceptor character of the substituents but also to the orbital symmetry of the atoms directly attached to the sp carbon atoms. Upon coordination, /(CC) for HC=CH decreases from 171.5 to 55.9 Hz.45... 

The chemical shifts of meta and para (but not ortho carbon atoms in monosubstituted benzenes correlate well with the inductive and resonance constants of the substituents " . Examination of and a° constants obtained from NMR data (Table 14) shows that for the groups containing a penta-coordinate silicon atom the cr-donor component is enhanced and the rc-acceptor one is lowered. Similar conclusions have been reached by other measurements e.g., the in-... 

An increase of the coordination number of the silicon atom is indicated by a highfield shift of the Si signal. However, there exists no relationship between the increasing chemical shifts and the increase in the coordinate interactions of the silicon atom. The substituents rather than any simple donor-acceptor inductive effects dominate the Si chemical coordination shift. 

The H and chemical shifts of the heterocyclic rings indicated that the silacycloalkyl substituents acted as r-acceptors. On the other hand, the hetaryl substituents influenced Si chemical shifts as n-donors. The Si chemical shifts of the compounds 1-8 appeared in much higher fields than those of corresponding 1,1-dimethyl derivatives [9, 10]. In both series they were shifted to the higher fields in the following order of substituents ... 

N chemical shifts and one-bond 15N—H coupling constants in the spectra of anilines show that electron-acceptor substituents in the ring favor the sp2 hybridization of the amine nitrogen atom20. 

Retcofsky and Griffin have outlined a method for determining Hammett substituent constants from chemical shift values in phenylphosphorus compounds (Table X). The values of the Hammett functions [Pg.175]

A study of substituent effects on the N chemical shift (5 N) (Table 10) for 4-substituted anilines in DMSO was interpreted in terms of substituent solvation-assisted resonance (SSAR) effects. Solvation of certain conjugated r-electron-acceptor (-I-R) substituents has been found to give signihcant enhancements in the acidities of anilines, phenols and other acids , and the magnitudes of these enhancements increase with... 

The preference of this conformation (the barrier hindering the rotation of the p-XCgH — at X = Cl, CHj, CFj amounts to 8.5 kcal/mol., cf. also ° 0 is assumed to be due, at least partially, to the donor-acceptor interaction between the K-system of the aryl residue and the carbenium centre at Cjg in the electronic absorption spectra of the 9-p-X-phenyl-9,10-dimethylphenanthrenium ions the charge transfer bands have been revealed whose position correlates well with the ionization potentials of respective X-substituted benzenes. Judging by the NMR- C spectra, however, the extent of this interaction in the main state of ions is insignificant — the chemical shift of the Cjo atom nearly remains unchanged as the X substituent... 

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