Motion of aromatic hydrocarbons in the microporous aluminum methylphosphonates AlMePO-α and AlMePO-β

2H wide-line NMR has been used, in conjunction with molecular dynamics simulations where appropriate, to follow the reorientation of the monoaromatic compounds benzene, toluene, and p-xylene within the one-dimensional channels of the α- and β-polymorphs of aluminum methylphosphonate, Al2(CH3PO3)3. Variable-temperature, static, 2H NMR spectra of adsorbed d6-benzene, d3-, d5-, and d8-toluenes, and d3,d3-p-xylene were matched by line shape simulation. The motion of p-xylene in both polymorphs is approximated by the long axis of the molecule describing a cone within the channels, the half-angle of which is greater for the slightly wider channels in AlMePO-β (27−30° cf. 18−19°). The 2H NMR of d3-toluene is simulated using a similar model, whereas the signal from aromatic deuterons in d5- and d8-toluenes is simulated by a ring undergoing 2π/3 flips around the para axis. The reorientation of benzene shows the largest differences between the two pore structures. In AlMePO-β it tumbles with little restriction, although at low temperatures the spectral details are better matched by allowing the molecule to spend a greater proportion of its time closer to the wall. In AlMePO-α the much broader line shape arises from constrained motion within the strongly triangular channels. Molecular dynamics simulations of benzene in the two structures confirm the differences. They support a model for benzene in AlMePO-α where its motion is restricted to rotations about its 6-fold axis and 2π/3 jumps between symmetry-related sites in the pores, so that the plane of the aromatic ring remains approximately parallel to the c-axis.