Chemical shift electronegativity effects

The decreased shielding caused by electronegative substituents is primarily an inductive effect and like other inductive effects falls off rapidly as the number of bonds between the substituent and the proton increases Compare the chemical shifts of the pro tons m propane and 1 mtropropane... 

If an H atom in an alkane R-// is replaced by a substituent X, the C chemical shift 8c in the a-position increases proportionally to the electronegativity of X (-/ effect). In the (1-position, Sc generally also increases, whereas it decreases at the C atom y to the substituent (y-effect, see Section 2.3.4). More remote carbon atoms remain almost uninfluenced (dSc 0). 

Substituent effects (substituent increments) tabulated in more detail in the literature demonstrate that C chemical shifts of individual carbon nuclei in alkenes and aromatic as well as heteroaromatic compounds can be predicted approximately by means of mesomeric effects (resonance effects). Thus, an electron donor substituent D [D = OC//j, SC//j, N(C//j)2] attached to a C=C double bond shields the (l-C atom and the -proton (+M effect, smaller shift), whereas the a-position is deshielded (larger shift) as a result of substituent electronegativity (-/ effect). 

The deshielding effects of electronegative substituents are cumulative, as the chemical shifts for various chlorinated derivatives of methane indicate. 

The chemical shifts for P—F compounds have been reviewed.The compounds differ from most other organophosphorus compounds because Sp becomes more positive as the electronegativity of the atoms attached to phosphorus increases. The effect is at a maximum for P" compounds. They behave normally with regard to an increase in shielding with increase in co-ordination number and therefore the P" compounds are the least shielded. Thus the largest negative values (— 190 to — 250) are observed for compounds of the type YPFj. With the new value of Sp of + 80 for PFg, the variation of Sp with the number of fluorine atoms in P compounds is now shown to be fairly consistent. The value of Sp has also been reported for a series of aminohalogeno P compounds. -... 

Among the earliest research on the transmission of substituent effects was the pioneering work of Lauterbur (58) and of Spiesecke and Schneider (59). For a wide series of substituted methanes and ethanes they found an approximate correlation of >3C chemical shifts with the electronegativity (E) of the substituents (X). So they concluded that inductive effects through o-bonds play an important role in determining the chemical shifts of a- and -positioned carbon atoms, and that this effect reflects the substituent s electron-withdrawing or -releasing ability. 

Any effect which alters the density or spatial distribution of electrons around a nucleus will alter the degree of shielding and hence its chemical shift. H chemical shifts are sensitive to both the hybridisation of the atom to which the H nucleus is attached sp, sp etc.) and to electronic effects (the presence of neighbouring electronegative/electropositive groups). 

There are excellent texts on proton chemical shifts listed in the references and this topic will not be discussed here. Specific aspects of substituent electronegativity, hyperconjugation and solvent effects are treated separately whenever a sufficient amount of data is available for the purpose of interpretation. 

A good variety of compounds have been prepared and characterized allowing us to obtain reliable information on the chemical shifts of saturated and unsaturated ring protons. A selection of different structures are shown in Table 4. The thiazine derivatives have been organized in groups for comparison. The effects of saturation (54, 105, 106), substituents of variable electronegativity (107, 108), increasing oxidation state of sulfur (121, 122, 27-29), and deprotonation (109-111) can be observed. 

Electronic effects that modify the polarity of a bond have an influence on chemical shifts. If the position of a methyl group in an alkyl halide CH3-X is compared to that of methane, the shift in signal increases with the electronegativity of the halogen atom (Table 9.3). 

From a study of a range of 2-substituted tetrahydropyrans it was concluded that the effects of an electronegative substituent were to deshield C-2 by about 30 p.p.m. and C-3 to a much reduced extent (ca. 4 p.p.m.), but to cause small upheld shifts in the signals from C-4, C-5 and C-6 (740MR(6)233). The influence of a 2-substituent on the chemical shift of C-4 can be used for quantitative conformational analysis provided suitable reference compounds are available. A comparatively large shielding of the C-4 signal indicates a preference of the 2-substituent for the axial position. 

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