Computational Studies of Load-Dependent Guest Dynamics and Free Energies of Inclusion for CO2 in Low-Density p-tert-Butylcalix[4]arene at Loadings up to 2:1

The structure, dynamics, and free energies of absorption Of CO2 by a low-density structure (P4/n) of calixarene p-tert-butylalix[4]arene (TBC4) at loadings up to 2:1 CO2:TBC4 have been studied by using molecular dynamics simulations with two sources of initial TBC4 structures (TBC4-T and TBC4-U). The CO2/TBC4 complex structure is very sensitive to the initial lattice spacing of TBC4. From the computed radial distribution functions Of CO2 molecules, a CO2 dimer is observed for TBC4-T and a cage-interstitial CO2 structure is suggested for TBC4-U. The dynamics of the CO2 molecules show little initial TBC4 structural dependency. The free energy of inclusion for a single CO2 in this TBC4 structure for various loadings is -4.0 kcal/mol at 300 K and -1.8 kcal/mol at 450 K, showing that CO2 inclusion is favored. The fully loaded 1:1 CO2:TBC4 system is slightly less favorable at -3.9 and - 1.2 kcal/mol at 300 and 450 K, respectively. The first CO2 added beyond 1:1 loading shows a significant drop in absorption energy to - 1.9 and + 1.9 kcal/mol at 300 and 450 K. These data are consistent with experimental results showing that low-density structures of TBC4 are able to absorb CO2 at loadings greater than 1:1 but retention is lower than for 1:1 loaded systems indicating the free energy of inclusion for addition of the CO2 above 1:1 is less favorable.