An Infrared νNH Scale for Weakly Basic Anions. Implications for Single-Molecule Acidity and Superacidity

The N−H stretching frequencies of tri-n-octylammonium salts are reported for a series of weakly
basic anions (A-), many of which are the conjugate bases of known strong acids and superacids. Data
have been collected primarily in carbon tetrachloride, where Oct3N+H···A- contact ion pairs are formed.
In the more polar solvent 1,2-dichloroethane, some salts form both contact and solvent-separated ion pairs.
Salts have also been studied in crystalline form or as oils. In general, the νNH frequencies decrease in the
order fluoroanions > carboranes > oxyanions, reflecting the relative basicities of the anions. By inference,
the data reflect differences in the acidity of the corresponding conjugate acids (HA). This qualitative indicator
of acid strength is useful because it reflects acidity on an individual molecule basis rather than in bulk. In
this respect, it provides a condensed-phase analogy to gas-phase (“intrinsic”) acidity and gives insight into
the aggregation phenomena that determine bulk acidity. The data also reveal the importance of the chemical
stability of conjugate base anions in attaining high acidity and suggest where acids stronger than those
presently known may be discovered.