Torsional Motion in (tert-Butyl)ammonium Hemispheraplexes: Rotational Barriers and Energy of Binding
✍ Scribed by Emily F. Maverick; Carolyn B. Knobler; Saeed Khan; James W. Canary; Ira B. Dicker; Kenneth N. Trueblood
- Publisher
- John Wiley and Sons
- Year
- 2003
- Tongue
- German
- Weight
- 162 KB
- Volume
- 86
- Category
- Article
- ISSN
- 0018-019X
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✦ Synopsis
Abstract
The ADPs (ADPs=atomic anisotropic displacement parameters) from the single‐crystal X‐ray studies of nine related TBA^+^ (TBA^+^=(tert‐butyl)ammonium) hemispheraplexes are analyzed, and the results compared to the free energy of binding of this guest by the nine hosts. The lipophilic hosts (Fig. 1) were synthesized over a number of years, with increasing pre‐organization for and specificity of binding. Structural studies for six of the complexes have been published, but the remaining three structures, including those of the strongest binders of TBA^+^, are disordered and have only now been completed.
New area‐detector data has been analyzed for the TBA^+^ClO$\rm{_{4}^{-}}$ complexes of 5 and of 8 at two temperatures, while the original data for 9⋅TBA^+^SCN^−^ has been treated with a disorder model. In addition, improved models are presented for the complexes of 6 and 7. Methods for assessing the precision of the ADP analyses are discussed. Although most of the structures are imprecise, the TBA^+^ groups do demonstrate some of the characteristics of independent motion. The general trend in calculated libration amplitudes for the TBA^+^ group suggests that the guests with the greatest free energy of binding, and the shortest distances from N^+^ to the ligand plane, are those with the highest barriers to internal rotation.