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+ClO4- 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.
ASJC Scopus subject areas
- Drug Discovery
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry