The p Hydrogen-Bond Basicity Scale: From Molecules to Anions.

The p (logarithm of complexation constant 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 on NBuX ion pairs in CCl, (ii) the estimation of from linear free energy relationships between measured values and literature values for various phenols in polar solvents, and (iii) the computation of at the density functional theory level in CCl. The scale extends on a 9.4 p unit range from fluoride to tetraphenylborate. Considering a number of anions as organic functions substituted with unipolar substituents, their p values can be related to the Hammett-Taft substituent constants σ. Unipolar substituents (O and S) obey the same p versus σ relationships as dipolar ionic (NNR) and dipolar (OH, CF, NR, 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.