The p K BHX 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.