Investigating the reasons for the significant influence of lattice flexibility on self-diffusivity of ethane in Zn(tbip)

Published molecular dynamics simulations of the self-diffusivity, DS, of ethane within the one-dimensional 4.5 Å channels of Zn(tbip) (H2tbip = 5-tert-butyl isophthalic acid) have shown not only quantitative, but also qualitative, differences in the DS values for fixed and flexible lattices when the concentration of molecules, c, is increased. The reasons for these differences are investigated with the aid of probability density plots, free energy landscapes and barriers, along with a determination of the structural changes accompanying increasing c. It is found that for flexible lattices, the tighter, more constrained parts of the channels become wider at higher c; this allows more molecules to diffuse in the central region of the channels.