144692-75-1Relevant academic research and scientific papers
Solvophobic and entropic driving forces for forming velcraplexes, which are four-fold, lock-key dimers in organic media
Cram, Donald J.,Choi, Heung-Jin,Bryant, Judi A.,Knobler, Carolyn B.
, p. 7748 - 7765 (2007/10/02)
Systems have been prepared composed of aromatic groups rigidly arranged to form ～ 15 by ～20 A rectangular surfaces containing two regularly spaced protruding methyls at 3 and 9 o'clock and two methyl-sized holes at 12 and 6 o'clock (e.g., 1). These molecules form dimers with large common surfaces in which the four methyl groups insert into the four holes. The resulting complexes have 82 to 132 intermolecular atom-to-atom contacts at van der Waals distances +0.2 ? (five crystal structures). Substitution of ethyls or hydrogens for the central aryl methyls eliminates complexation. Eight substituents attached at the periphery of the monomers extend the surfaces and profoundly affect the binding free energies (-ΔG° values) of the complexes, which range from 9 kcal mol-1 in CDCl3 at temperatures -30 to 25 °C. Peripheral substituents with rotational degrees of freedom inhibit homodimerization. Activation free energies (ΔG) for five dimerizations ranged from 8 to 10 kcal mol-1 and for five dissociations from 10 to 15 kcal mol-1 (in CDCl3), suggesting their transition states to be very poorly solvated. The -ΔG° values for dimerization with notable exceptions increased dramatically with solvent polarity and polarizability. Enthalpies (ΔH values) ranged from +6 to -8 kcal mol- and entropies (ΔH values) from -6 to +40 cal mol-1 K-1. Some dimerizations were entropy driven and enthalpy opposed, pointing to large solvophobic effects in organic media.
