title: 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.
abstract: The structure, dynamics, and free energies of absorption of CO(2) by a low-density structure (P4/n) of calixarene p-tert-butylalix[4]arene (TBC4) at loadings up to 2:1 CO(2):TBC4 have been studied by using molecular dynamics simulations with two sources of initial TBC4 structures (TBC4-T and TBC4-U). The CO(2)/TBC4 complex structure is very sensitive to the initial lattice spacing of TBC4. From the computed radial distribution functions of CO(2) molecules, a CO(2) dimer is observed for TBC4-T and a cage-interstitial CO(2) structure is suggested for TBC4-U. The dynamics of the CO(2) molecules show little initial TBC4 structural dependency. The free energy of inclusion for a single CO(2) 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 CO(2) inclusion is favored. The fully loaded 1:1 CO(2):TBC4 system is slightly less favorable at -3.9 and -1.2 kcal/mol at 300 and 450 K, respectively. The first CO(2) 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 CO(2) 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 CO(2) above 1:1 is less favorable.
authors.affiliation: Pacific Northwest National Laboratory, P.O. Box 999, Richland, Washington 99352, USA.
authors.forename: John L
authors.initials: JL
authors.lastname: Daschbach
authors.forename: Xiuquan
authors.initials: X
authors.lastname: Sun
authors.forename: Tsun-Mei
authors.initials: TM
authors.lastname: Chang
authors.forename: Praveen K
authors.initials: PK
authors.lastname: Thallapally
authors.forename: B Peter
authors.initials: BP
authors.lastname: McGrail
authors.forename: Liem X
authors.initials: LX
authors.lastname: Dang