title: Motion of aromatic hydrocarbons in the microporous aluminum methylphosphonates AlMePO-alpha and AlMePO-beta.
abstract: (2)H 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)(CH(3)PO(3))(3). Variable-temperature, static, (2)H 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 AlMePO-beta (27-30 degrees cf. 18-19 degrees). The (2)H 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 2pi/3 flips around the para axis. The reorientation of benzene shows the largest differences between the two pore structures. In AlMePO-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 AlMePO-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 AlMePO-alpha where its motion is restricted to rotations about its 6-fold axis and 2pi/3 jumps between symmetry-related sites in the pores, so that the plane of the aromatic ring remains approximately parallel to the c-axis.
authors.affiliation: School of Chemistry, University of St. Andrews, St. Andrews, Fife, United Kingdom.
authors.forename: Jorge
authors.initials: J
authors.lastname: Gonzalez
authors.forename: R Nandini
authors.initials: RN
authors.lastname: Devi
authors.forename: Paul A
authors.initials: PA
authors.lastname: Wright
authors.forename: David P
authors.initials: DP
authors.lastname: Tunstall
authors.forename: Paul A
authors.initials: PA
authors.lastname: Cox
mesh_terms: D000535:Aluminum / Q000737:chemistry*; D001554:Benzene / Q000737:chemistry*; D018360:Crystallography, X-Ray; D009682:Magnetic Resonance Spectroscopy / Q000379:methods; D008956:Models, Chemical; D008958:Models, Molecular; D009942:Organometallic Compounds / Q000737:chemistry*; D009943:Organophosphorus Compounds / Q000737:chemistry*; D016062:Porosity; D012680:Sensitivity and Specificity; D014050:Toluene / Q000737:chemistry*; D014992:Xylenes / Q000737:chemistry*
chemical_list: D009942:Organometallic Compounds; D009943:Organophosphorus Compounds; D014992:Xylenes; D014050:Toluene; C031286:4-xylene; D000535:Aluminum; D001554:Benzene