Motion of Aromatic Hydrocarbons in the Microporous Aluminum Methylphosphonates AlMePO-α and AlMePO-β

H-2 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 alpha- and beta-polymorphs of aluminum methylphosphonate, Al-2(CH3PO3)(3). Variable-temperature, static, H-2 NMR spectra of adsorbed d(6)-benzene, d(3)-, d(5)-, and d(8)-toluenes, and d(3),d(3)-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 A1MePO-beta (27-30 degrees cf. 18-19 degrees). The H-2 NMR of d(3)-toluene is simulated using a similar model, whereas the signal from aromatic deuterons in d(5)- and d(8)-toluenes is simulated by a ring undergoing 2:63 flips around the para axis. The reorientation of benzene shows the largest differences between the two pore structures. In A1MePO-beta 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 A1MePO-alpha 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 A1MePO-alpha where its motion is restricted to rotations about its 6-fold axis and 2 pi/3 jumps between symmetry-related sites in the pores, so that the plane of the aromatic ring remains approximately parallel to the c-axis.