Diffusion of vinyl bromide through the crystals of p-But-calix[4]arene

The mechanism for the diffusion of vinyl bromide (VB) through the nonporous van der Waals organic solid of p-tert-butylcalix[4]arene (tBC) is examined by molecular mechanics calculations. In a "squeeze" mechanism, a VB molecule passes through the interstitial space in the rim regions of tBC bowls, after the partial filling of nearby inclusion sites, and couples with the both the rotation of butyl groups and the translational sliding of the bilayers, until it encounters and falls into a empty site. In a "relay" mechanism, translational sliding shifts the skew angle between two tBC bowls and makes it possible to pass a VB from one bowl to another and throughout the solid. The barrier for the "relay" mechanism is 4 kcal/mol lower than the "squeeze" mechanism, although the "squeeze" mechanism is favored by entropy. The "squeeze" mechanism should be more sensitive to the size of the diffusion molecule and should leave residue VB molecules in the interstitial regions. In contrast, the VB molecule hops from one site to another in the "relay" mechanism, and the kinetically limiting step is the translational sliding of the bilayer. Such contrasts should help resolve the relative importance of the two mechanisms in future kinetic studies.