An infrared vNH scale for weakly basic anions.: Implications for single-molecule acidity and superacidity
An infrared vNH scale for weakly basic anions.: Implications for single-molecule acidity and superacidity
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DOI：
10.1021/ja060714v
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发表时间：
2006-07-05
影响因子：
15
通讯作者：
Reed, Christopher A.
中科院分区：
文献类型：
--
作者：
Stoyanov, Evgenii S.;Kim, Kee-Chan;Reed, Christopher A.
报道了一系列弱碱性阴离子（a -）的N - H伸展频率，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，其中形成了Oct3N+H···a接触离子对。在极性更强的溶剂1,2-二氯乙烷中，一些盐既形成接触离子对，也形成溶剂分离离子对。盐也以结晶形式或油的形式被研究过。一般来说，νNH频率在氟阴离子b>碳硼烷b>氧阴离子的顺序中减小，反映了阴离子的相对碱度。由此推断，这些数据反映了相应共轭酸（HA）的酸度差异。这种酸强度的定性指标是有用的，因为它反映了单个分子的酸度，而不是整体的酸度。在这方面，它提供了一个与气相（“本征”）酸度类似的冷凝相，并深入了解了决定整体酸度的聚集现象。这些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，并建议在哪里可以发现比目前已知的酸更强的酸。
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.