Chemical shifts substituent

Recently. Fourier transform technique allowed the determination in natural abundance of C chemical shifts for some 4-thiazoline-2-thiones. Substituent chemical shifts for methyl and phenyl groups have been collected and discussed (874). For the overcrowded polyalkyl-A-4-thiazoline-2-thiones. the evolution of these chemical shifts furnishes... 

Fluorine Chemical Shift Substituent Chemical Shift (SCS) ... 

Another important component of Table 18 is substituent chemical shift (SCS) datadenvedforeachoftheSlentnes The SCS is simply thedifferencein F-NMR chemical shifts of the substituted compounds and that of unsubstituted fluoroben zene (-113 5 ppm in CDCI3) These values numerically represent the mfluence a substituent has on the shieldmg or deshieldmg of the fluorine nucleus and depend upon substituent position o, m, orp) Fluonne chemical shifts can be predicted for polysubstituted fluorobenzene systems simply by addmg the SCS value of each substituent to a base value of -113 5 ppm... 

Finally, in this section, we mention that Adcock and colleagues117 have determined o, and 6R values for several substituents including S02Me by studies of substituent chemical shifts in fluoronaphthalenes, and parallel studies with fluorobenzenes, in several solvents. The results are summarized in Table 10. The most significant finding appears to be the diminution in oR values as between the fluorobenzene and fluoronaphthalene probe for... 

In the ortho positions, the substituent chemical shifts are between those for a double and a single bond, as would be expected on the basis of an aromatic bond. For these positions proximate interactions such as electric field effects assume an increased importance. 

N substituent chemical shifts (SCS) of a variety of anticonvulsant phenylacetanilides 1 and 2, with a substituent at the para or meta position of the aniline moiety, were analysed by means of DSP (dual substituent parameter) equations33. 

The emphasis in the foregoing parts of this chapter has been deliberately chemical . We have tried to explore the role of substituent constants in relation to understanding the effect of structure on reaction rates and equilibria, with particular reference to the NO2 group as a substituent. This chemical emphasis will continue in the later parts of the chapter, for NO2 and for the other substituents with which we are concerned, but in the present section there will be a change. In Section H.B brief reference was made to the use of substituent constants in the correlation analysis of spectroscopic data, particularly 19F and 13C substituent chemical shifts and infrared frequencies and intensities. These matters must now be explored in greater detail. 

There is a continual tendency for the values of cr/ and aR (and other substituted benzenes in very dilute solution in cyclohexane, carbon tetrachloride or deuteriochloroform were the basis for a redefinition of the aR scale and some amendment of a R values. However, the value for the nitro group was confirmed as 0.15. 

Reynolds and coworkers156 based a similar operation on 13C substituent chemical shifts of meta- and para-substituted styrenes. Iterative multiple regression was used for the redefinition of the 07 and crjj scales. The authors also took the opportunity to replace the symbol 07 by op, having become convinced that the so-called inductive effect was entirely a field effect (see the present author s discussion of this matter76). The authors presented an extensive table in which their values of the substituent parameters are compared with those obtained by other authors. Their cr value of 0.144 for NO2 essentially confirmed the various values of about 0.15 already mentioned and their 07 value of 0.651 essentially confirmed the various values of about 0.65. 

Happer157 determined 13C substituent chemical shifts for meta- and para-substituted styrenes in seven different solvents. Data for the side-chain carbons, and in the meta series for the ring carbon para to the substituent, were analyzed as a basis for assessing solvent effects on 07, cr, ar (BA) and cr. The 07 values for NO2 varied from 0.60 in DMSO to 0.71 in CDCI3 in EtOH the value was 0.64. The [Pg.505]

Structure type Ring system Substituents Chemical shifts (5, ppm) Ref... 

The chemical shift <5 of a given nucleus in a molecule contains stereochemical information which, however, might often be uninterpretable because the corresponding chemical shift of another stereoisomer is not available for comparison. Thus, the evaluation of an isolated chemical shift is possible only if the contribution of the stereochemical influence can be identified unequivocally among other influences. This is, for example, the case when substituent effects on chemical shifts (SCS substituent chemical shift), instead of chemical shifts themselves, are determined, since some of these values are known to possess a well-defined stereochemical dependence. An SCS is defined as the chemical shift of a given nucleus in a substituted molecule (R—X) relative to the unsubstituted parent compound (R H) ... 

Table 14 13C Substituent Chemical Shift Parameters for Pyrrole Derivatives 1...

Although most substituted coumarins give rise to signals which agree with those calculated by the normal additivity of substituent chemical shifts, deviations from the predicted values are observed for C-6 and C-7 and for the carbon atoms of the substituents in 6,7-dimethylcoumarin (8IT2021). These ortho proximity effects have been noted in aromatic... 

Adcock and Kok have analysed the 19F chemical shifts of a range of 3-substituted 1-fluoroadamantane derivatives 62. The substituent chemical shift (19F SCS) can be factorized into polar field (pf f) and residual contributions (19F SCS - pf f)- A significant solvent-independent residual contribution is observed when R = SiMe3, and is attributed to homohyperconjugation70. 

In Section n.B brief reference was made to the use of substituent constants in the correlation analysis of spectroscopic data, particularly 19F and 13C substituent chemical shifts and infrared frequencies and intensities. These matters must now be explored in greater detail as background to later discussions of individual substituents relevant to the present chapter. 

Reynolds and coworkers48 based a similar operation on 13C substituent chemical shifts of meta- and para-substituted styrenes. Iterative multiple regression was used for the redefinition of the 0/ and ap scales. The authors also took the opportunity to replace the... 

Other estimates of inductive and resonance parameters for these groups have been based on the correlation analysis of 13C substituent chemical shifts, e.g. in the work of Reynolds... 

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