A1 Refereed original research article in a scientific journal
Translation and reorientation of CD(4) molecules in nanoscale cages of zeolites as studied by deuteron spin-lattice relaxation
Authors: Birczynski A, Punkkinen M, Szymocha AM, Lalowicz ZT
Publisher: AMER INST PHYSICS
Publication year: 2007
Journal:: Journal of Chemical Physics
Journal name in source: JOURNAL OF CHEMICAL PHYSICS
Journal acronym: J CHEM PHYS
Article number: ARTN 204714
Volume: 127
Issue: 20
Number of pages: 10
ISSN: 0021-9606
DOI: https://doi.org/10.1063/1.2790903
Abstract
Deuteron spin-lattice relaxation was applied to study translational and rotational mobility of CD(4) molecules trapped in the cages of zeolites. Tetrahedral methane molecules are treated as quantum rotators. Relaxation rates related to the intraquadrupole interaction are derived for the T and A+E symmetry species in the presence of large tunneling splittings, consistently with the assumption that A and E species molecules relax at the same rate. An exchange model is presented, which describes the effect on relaxation of CD(4) jumping between two positions characterized by different potentials. While staying at either position bonded to an atom or ion at the cage wall, the molecule has some freedom to move in the vicinity. This causes a time-dependent external electric field gradient, which contributes to the deuteron relaxation rate via the electric quadrupole interaction. Spin conversion transitions couple the relaxation of magnetizations M(T) and M(AE), which is taken into account by reapplying the presented model under somewhat different conditions. Such a two-step procedure leads to successful fits with the experimental results obtained in the range of temperatures roughly 20-200 K for zeolites HY, NaA, and NaMordenite. At higher temperatures CD(4) molecules fly freely across zeolite cages and relaxation changes accordingly, while incoherent tunneling dominates for immobile molecules below 20 K. (c) 2007 American Institute of Physics.
Deuteron spin-lattice relaxation was applied to study translational and rotational mobility of CD(4) molecules trapped in the cages of zeolites. Tetrahedral methane molecules are treated as quantum rotators. Relaxation rates related to the intraquadrupole interaction are derived for the T and A+E symmetry species in the presence of large tunneling splittings, consistently with the assumption that A and E species molecules relax at the same rate. An exchange model is presented, which describes the effect on relaxation of CD(4) jumping between two positions characterized by different potentials. While staying at either position bonded to an atom or ion at the cage wall, the molecule has some freedom to move in the vicinity. This causes a time-dependent external electric field gradient, which contributes to the deuteron relaxation rate via the electric quadrupole interaction. Spin conversion transitions couple the relaxation of magnetizations M(T) and M(AE), which is taken into account by reapplying the presented model under somewhat different conditions. Such a two-step procedure leads to successful fits with the experimental results obtained in the range of temperatures roughly 20-200 K for zeolites HY, NaA, and NaMordenite. At higher temperatures CD(4) molecules fly freely across zeolite cages and relaxation changes accordingly, while incoherent tunneling dominates for immobile molecules below 20 K. (c) 2007 American Institute of Physics.
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