Protons, shielding/deshielding

FIGURE 13 10 More shielded (red) and less shielded (blue) protons in (a) [18]annulene and (b) [16]annulene The induced magnetic field associated with the aromatic ring current in [18]annulene shields the inside protons and deshields the out side protons The opposite occurs in [16]annulene which is antiaromatic... 

The chemical shift is determined by the extent to which a proton is deshielded by the groups to which it is attached. The more a proton is shielded by the electron density around it, the lower its 3 value. If a proton is attached to a system that withdraws electrons from its environment such as an electronegative group or to a group which affects its environment by creating a field opposing the applied... 

If all protons were shielded by the same amount, they would all be in resonance at the same combination of frequency and magnetic field. Fortunately, protons in different chemical environments are shielded by different amounts. In methanol, for example, the electronegative oxygen atom withdraws some electron density from around the hydroxyl proton. The hydroxyl proton is not shielded as much as the methyl protons, so the hydroxyl proton absorbs at a lower field than the methyl protons (but still at a higher field than a naked proton). We say that the hydroxyl proton is deshielded somewhat by the presence of the electronegative oxygen atom. 

Substituents which shield the H3 4 protons and deshield the H2 5 protons. 

The thiocyanate group has an intermediate deshielding effect on adjacent aliphatic groups but little or no shielding/deshielding effect on the aromatic protons. 

The 1-phenyl ureas display an unusual type of aromatic shielding, i.e. the para proton is shielded while the ortho protons are deshielded. This shielding is not observed when two phenyl groups are bonded to the same nitrogen atom. In this case, a single peak is often observed near 7.3 ppm. 

The chemical shift of any given proton depends upon the combination of effects, which are (roughly) additive the effects reinforce or cancel one another. Thus, acetylenic protons are deshielded by the inductive effect (acetylene is acidic) but shielded by the anisotropy of the triple bond a value of S = 1.80 ppm results. 

Table 20.2 shows typiced vedues of chemical shifts for protons in organic compounds. Notice that protons attached to a carbon atom also bonded to a halogen tend to have relatively high chemical shift (8) values the electronegative atoms reduce the electron density around the proton so that it experiences more of the external magnetic field (the proton is deshielded) and absorbs radiation at a higher frequency. Similarly, if a group or atom pushes electron density on to a proton (shields it), the proton absorbs radiation at a lower frequency (its chemical shift is lower). 

---