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

The p K BHX (logarithm of complexation constant K of 4-fluorophenol with 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) the infrared measurement of K on NBu 4 + X – ion pairs in CCl 4, (ii) the estimation of K from linear free energy relationships between measured K values and literature K values for various phenols in polar solvents, and (iii) the computation of K at the density functional theory level in CCl 4 . The scale extends on a 9.4 p K unit range from fluoride to tetraphenylborate. Considering a number of anions as organic functions substituted with unipolar substituents, their p K BHX values can be related to the Hammett–Taft substituent constants σ. Unipolar substituents (O – and S – ) obey the same p K BHX versus σ relationships as dipolar ionic (N – N + R 3 ) and dipolar (OH, CF 3, NR 2, or OR) ones for the nitrile, carbonyl, nitroso, nitro, sulfonyl, and phosphoryl functions. Like dipolar substituents, unipolar substituents at carbon and nitrogen operate by field-inductive and resonance effects, whereas substituents at sulfur and phosphorus operate only by the field-inductive effect.