(284a) Progression of Conformational Transitions of Alkanes Under Hydrophobic Confinement in Deep-Cavity Cavitand Dimer Complexes

Simulations of guest-host systems give an opportunity for studying confinement-induced conformational changes. One particlar host, octa-acid (OA), a bowl-shaped molecule, is useful in separations and nanoreactions. The hydrophobic effect drives OA to form dimers around specific hydrophobic solutes, allowing for study of the hydrophobic guest's structural conformations under confinement. A recent NMR study suggests that linear alkanes exhibit specific length-dependent guest-packing motifs when encapsulated in an OA dimer, from an extended, to helical, to j-turn, to spinning top structure with increasing chain length. In this study we use Molecular Dynamics simulations of these systems to get a level of structural detail not possible with NMR alone. By utilizing principal components analysis we are able to wittle down the vast about of simulation data in order to identify the guest-packing motifs present and perform a direct comparison of computed isotropic shielding constants with experimental NMR chemical shifts of the bound guests. These structures show a clear progression of guest-packing motifs as the guest's length increases. To further characterize the motifs, free energy difference calculations of growing guests inside the capsule were performed, giving another interpretation to the progression of motifs. These results not only affect our understanding of how guest size affects packing motif, but they may also aid in the engineering of capsular molecular hosts and their guests.