Chemical shift electronegativity

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... 

The most obvious feature of these C chemical shifts is that the closer the carbon is to the electronegative chlorine the more deshielded it is Peak assignments will not always be this easy but the correspondence with electronegativity is so pronounced that spec trum simulators are available that allow reliable prediction of chemical shifts from structural formulas These simulators are based on arithmetic formulas that combine experimentally derived chemical shift increments for the various structural units within a molecule... 

NMR The electronegative oxygen of an alcohol decreases the shielding of the car bon to which it is attached The chemical shift for the carbon of the C—OH is 60-75 ppm for most alcohols Carbon of a C—S group is more shielded than carbon of C—O... 

H NMR The chemical shift of the proton m the H—C—O—C unit of an ether is very similar to that of the proton m the H—C—OH unit of an alcohol A range of 8 3 2-4 0 IS typical The proton m the H—C—S—C unit of a sulfide appears at higher field than the corresponding proton of an ether because sulfur is less electronegative than oxygen... 

Section 16 18 An H—C—O—C structural unit m an ether resembles an H—C—O—H unit of an alcohol with respect to the C—O stretching frequency m its infrared spectrum and the H—C chemical shift m its H NMR spectrum Because sulfur is less electronegative than oxygen the H and chemical shifts of H—C—S—C units appear at higher field than those of H—C—O—C... 

The chemical shift is related to the part of the electron density contributed by the valence electrons, ft is a natural extension, therefore, to try to relate changes of chemical shift due to neighbouring atoms to the electronegativities of those atoms. A good illustration of this is provided by the X-ray photoelectron carbon Is spectmm of ethyltrifluoroacetate, CF3COOCH2CH3, in Figure 8.14, obtained with AlXa ionizing radiation which was narrowed with a monochromator. 

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. 

Table 13.3 shows the correlation of H chemical shift with electronic environment in more detail. In general, protons bonded to saturated, sp3-hybridized carbons absorb at higher fields, whereas protons bonded to s/ 2-hybridized carbons absorb at lower fields. Protons on carbons that are bonded to electronegative atoms, such as N, O, or halogen, also absorb at lower fields. 

There have, however, been attempts to correlate Q-e values and hence reactivity ratios to, for example, c NMR chemical shifts 50 or the results of MO calculations 51153 and to provide a better theoretical basis for the parameters. Most recently, Zhan and Dixon153 applied density functional theory to demonstrate that Q values could be correlated to calculated values of the relative free energy for the radical monomer reaction (PA + Mn — PA ). The e values were correlated to values of the electronegativities of monomer and radical. 

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