Chemical shift electronic effects

Chemical shift Electrons of the atoms and molecules surrounding a nucleus interact with B0 and induce an additional local field at the position of the nucleus being probed. The effect of this local magnetic field is to reduce the magnitude of the external magnetic field experienced by local nuclei. This results in a shift in the resonance frequency of nuclei. Chemical shifts are measured in parts per million (ppm). 

Be able to identify simple organic molecules from their H NMR spectra through the interpretation of their integrals (the number of hydrogens in each environment), chemical shifts (the effect of functional groups on the electron density of the molecule) and coupling patterns (the number of spin active nuclei on adjacent atoms)... 

As is typical of all inductive effects, this type of deshielding decreases rapidly with increasing distance from the electronegative atom. Thus, the methyl group of ethyl chloride resonates at 8 = 1.33 ppm. In general, factors influencing electron density in the proximity of the proton are reflected in the chemical shift. Electron deficiency is associated with deshielding and therefore results in downfield shifts from TMS. 

Making allowance for those effects gives a good correlation between the chemical shifts and the it- and/or tr-electron density of the carbon atom bearing the proton (133, 236,237). 

The left-hand side of Equation (8.15) involves the difference between two electron binding energies, E — E. Each of these energies changes with the chemical (or physical) environment of the atom concerned but the changes in Ek and E are very similar so that the environmental effect on Ek — E is small. It follows that the environmental effect on E -h Ej, the right-hand side of Equation (8.15), is also small. Therefore the effect on is appreciable as it must be similar to that on There is, then, a chemical shift effect in AES rather like that in XPS. 

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. 

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). 

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