Resonance effects from functional groups, Table

Table 4.8 Resonance effects from functional groups...

For a portion of their time, these molecules will be oriented with their Tt-bonds perpendicular to applied field, as illustrated in Figure 6. The current distribution of the 7i-electrons is such that a magnetic field is produced that opposes the applied field above and below the bond, but reenforces the applied field at the attached proton. An exception to this is the terminal alkyne group (Figure 6), where the proton is oriented in the same plane as the rt-bond. The major contribution comes when the 7t-bond is oriented parallel to the applied field. The current distribution from the 7i-electrons leads to an induced field that opposes the applied field at the proton. All of the above effects are additive in determining the final chemical shift of a proton and the position of resonances can be suggestive of specific function groups (see Table 1). 

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