Hydrogen‐bonding. Part 4. An analysis of solute hydrogen‐bond basicity, in terms of complexation constants (log K), using F₁ and F₂ factors, the principal components of different kinds of basicity

The principal components factors F 1 and F 2 in the equation have been used to obtain S 1 and S 2 values for sets of hydrogen‐bond bases against 32 reference acid/solvent systems. The constants S 1 and S 2 define an angle θ = tan −1 S 2 / S 1 that is a measure of the electrostatic:covalent bonding ratio in the hydrogen‐bond complex. It is shown that θ can vary from 53 (4‐fluorophenol in CH 2 Cl 2 )to 86 degrees (Ph 2 NH in CCl 4 ) depending on the reference acid and solvent. This variation in θ can lead to family dependent behaviour in plots of log K for bases against a given reference acid system vs log K for bases against another reference acid system, and precludes the construction of any general scale of hydrogen‐bond basicity using log K values. Amongst a quite wide range of reference acid/solvent systems θ varies only from 64 to 73 degrees, and for bases against these reference systems a ‘reasonably general’ scale could be set up. Such a scale could be extended to bases against reference acid/solvent systems outside the 64–73 degree range provided that certain classes of base (e.g. pyridines, alkylamines) were excluded from the additional reference acid/solvent systems.