(773a) Deep Cavity Cavitand/Alkane Assembly State Switching between Monomeric and Dimeric Host:Guest Assemblies Driven By Guest Packing

Deep
Cavity Cavitand/Alkane Assembly State Switching Between Monomeric and Dimeric
Host:Guest Assemblies Driven by Guest Packing

Du
Tang^†, J. Wesley Barnett^†, Bruce C. Gibb^‡, and
Henry S. Ashbaugh^†

†
Department of
Chemical and Biomolecular Engineering, Tulane University, New Orleans, LA

‡ Department of Chemistry,
Tulane University, New Orleans, LA

Octa-acid
(OA) and tetra-endo-methyl octa-acid (TEMOA) are water-soluble, deep-cavity
cavitand hosts with nanometer-sized hydrophobic pockets that readily bind
nonpolar guests, like the n-alkanes. Experimentally the host-guest
stoichiometry in these assemblies depends sensitively on the length of
encapsulated n-alkanes. OA, for instance, exhibits a progression of 1:1 to 2:2
to 2:1 host-guest complexes (X:Y where X is the number of hosts and Y is the
number of guests) with increasing alkane chain length from methane to
tetradecane. Despite differing from OA by only the addition of 4 methyl groups,
TEMOA exhibits a non-monotonic progression of states from 1:1 to 2:2 to 1:1 to
2:1 with increasing guest length. Understanding the interplay between host
chemistry and the equilibrium structures formed can impact the potential
applications for these supramolecular assemblies. Here we present a molecular
simulation study of n-alkane association with both OA and TEMOA to form the
full range of structures observed in an effort to uncover the reasons
underlying their distinct assemblies. Potentials-of-mean force between hosts
and guests, determined via umbrella sampling, are used to characterize their
association free energies. The free energies so determined are subsequently
used within the context of a reaction network model in order to rationalize the
sequence of assemblies formed. Our modeling results show that TEMOA’s added
methyl units constrict the opening of the binding pocket, limiting the
conformations available to encapsulated guests and thereby shifting the balance
between monomeric and dimeric capsules.