Stoyanov Evgenii S, Kim Kee-Chan, Reed Christopher A
Department of Chemistry, University of California, Riverside, CA 92521, USA.
J Am Chem Soc. 2006 Jul 5;128(26):8500-8. doi: 10.1021/ja060714v.
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 Oct(3)N(+)-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 nuNH 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.
报道了一系列弱碱性阴离子（A(-)）的三正辛基铵盐的N-H伸缩频率，其中许多是已知强酸和超强酸的共轭碱。数据主要在四氯化碳中收集，在那里形成了Oct(3)N(+)-H...A(-)接触离子对。在极性更强的溶剂1,2-二氯乙烷中，一些盐同时形成接触离子对和溶剂分隔离子对。盐也以晶体形式或油状进行了研究。一般来说，νNH频率按氟阴离子>碳硼烷>氧阴离子的顺序降低，反映了阴离子的相对碱性。由此推断，这些数据反映了相应共轭酸（HA）酸度的差异。这种酸强度的定性指标很有用，因为它反映的是单个分子的酸度，而不是整体的酸度。在这方面，它提供了与气相（“本征”）酸度的凝聚相类比，并深入了解了决定整体酸度的聚集现象。数据还揭示了共轭碱阴离子的化学稳定性在获得高酸度方面的重要性，并指出了可能发现比目前已知更强酸的地方。
