Spectroscopic Scales of Basicity Affinity

Besides direct measurement of the thermodynamic quantities K and AH discussed above, spectroscopic estimates of basicity (affinity) have been proposed. Their main attraction is the ease with which they can be carried out. Moreover, many systems that do not possess the physical properties needed for a thermodynamic study can be characterized by spectroscopic parameters. Spectroscopic scales of basicity (affinity) are based on the change of a spectrochemical property (NMR, UV-Vis, IR, etc.) of the Lewis acid upon complexation 

NMR data [116,121 ] for the limiting F chemical shift of 4-fluorophenol upon hydrogen bonding with Lewis bases fulfil these conditions  

The last shortcoming concerns the relationships between spectroscopic and thermodynamic scales of basicity (affinity). It appears that they often have a more limited validity domain than claimed by their advoeates. As shown in Chapters 4 and 5, these relationships are often family dependent, that is, they are limited to bases that are structurally related. In the correlation of two basieity-dependent properties, it is often found that the organic bases separate into families that are distinguished by their basic sites as follows  

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