The pKBHX Hydrogen-Bond Basicity Scale: From Molecules to Anions

The pKBHX(logarithm of complexation constantKof 4-fluorophenolwith bases) hydrogen-bond basicity scale of neutral hydrogen-bond acceptors (HBAs)is extended to anionic HBAs. The scale is constructed for 26 anions through (i) theinfrared measurement ofKon NBu4+X-ion pairs in CCl4, (ii) the estimation ofKfromlinear free energy relationships between measuredKvalues and literatureKvalues forvarious phenols in polar solvents, and (iii) the computation ofKat the densityfunctional theory level in CCl4. The scale extends on a 9.4 pKunit range fromfluorideto tetraphenylborate. Considering a number of anions as organic functions substitutedwith unipolar substituents, their pKBHXvalues can be related to the Hammett-Taftsubstituent constants Sigma. Unipolar substituents (O-and S-) obey the same pKBHXversus Sigma relationships as dipolar ionic (N-N+R3) and dipolar (OH, CF3,NR2, or OR) ones for the nitrile, carbonyl, nitroso, nitro, sulfonyl,and phosphoryl functions. Like dipolar substituents, unipolar substituents at carbon and nitrogen operate byfield-inductive andresonance effects, whereas substituents at sulfur and phosphorus operate only by thefield-inductive effect.

Automated summary

The pKBHX hydrogen-bond basicity scale is extended to anionic HBAs, allowing the evaluation of anions' basicity. The scale is constructed through experimental measurements, linear free energy relationship estimations, and density functional theory calculations. The scale also reveals a correlation between the basicity of anions with unipolar substituents and the Hammett-Taft substituent constants Sigma.