College Park, Maryland June 6 - 10 , 2004
M2-D2 (11:00 AM): Methyl Rotational Tunneling Dynamics of p-xylene confined in a crystalline zeolite host
S. Nair (School of Chemical & Biomolecular Engineering, Georgia Tech), R. M. Dimeo, D. A. Neumann (NIST Center for Neutron Research), A. J. Horsewill (Department of Physics & Astronomy, University of Nottingham)
The rotational tunneling of molecules, functional groups or ions is known to be a sensitive and quantitative probe of the local energetics and structure of a material. Crystalline nanoporous aluminosilicates (zeolites) are used in organic separations owing to their shape- and size-selectivity for various types of organic sorbate molecules. This behavior is influenced by the framework-sorbate and sorbate-sorbate interactions, which determine the molecular adsorption and transport processes in the porous material.The methyl rotational tunneling spectrum of p-xylene confined in nanoporous zeolite crystals has been measured by inelastic neutron scattering (INS) and proton nuclear magnetic resonance (NMR), and analyzed to extract the rotational potential energy surfaces characteristic of the methyl groups in the host-guest complex. The number and relative intensities of the tunneling peaks observed by INS indicate the presence of methyl-methyl coupling interactions in addition to the methyl-zeolite interactions. The INS tunneling spectra from the crystals (space group P212121 with four crystallographically inequivalent methyl rotors) are quantitatively interpreted as a combination of transitions involving two coupled methyl rotors as well as a transition involving single-particle tunneling of a third inequivalent rotor, in a manner consistent with the observed tunneling energies and relative intensities. Together, the crystal structure and the absence of additional peaks in the INS spectra suggest that the tunneling of the fourth inequivalent rotor is strongly hindered and inaccessible to INS measurements. This is verified by proton NMR measurements of the spin-lattice relaxation time which reveal the tunneling characteristics of the fourth inequivalent rotor.
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