MOLECULAR-DYNAMICS SIMULATION STUDIES OF ZEOLITE-A .4. STRUCTURE AND DYNAMICS OF NH4-A( IONS IN A RIGID DEHYDRATED ZEOLITE)

In recent papers [1-3] we reported molecular dynamics simulation studi es of ions and water molecules adsorbed in a rigid zeolite-A framework using a simple Lennard-Jones potential plus Coulomb potential with Ew ald summation to investigate the structure and dynamics of the adsorba tes. In the present paper the same technique is applied to study the l ocal structure and dynamics of NH4+ ions in a rigid dehydrated zeolite -A. During the preliminary equilibration, the unstable NH4(4) type ion (the 12th ion) is pushed down to near a more stable 6-ring position i n the alpha-cage that is already associated with an NH4(1) type ion (t he 1st) in the beta-cage, which moves to another 6-ring position in th e beta-cage that is already associated with an NH4(2) type ion (the 7t h) in the alpha-cage. Calculated x, y, and z coordinates of some NH io ns are in good agreement with those obtained from an X-ray diffraction experiment except that no NH4(4) type ion is found and there are six NH4(2) type ions instead of 0.5 and 5.5 occupancy. The analyses of cal culated interatomic distances and time correlation functions of these ions indicate that the NH4(1--1) and NH4(3) type ions are associated l oosely with only one O (3) atom of the 6-ring and with only one O(1) a tom of the 8-ring windows, respectively, while the NH4(1-2) and NH4(2) type ions are associated strongly with two or three O (3) atoms of th e 6-ring windows in the alpha- and beta-cages, respectively. The analy sis of hydrogen bond time correlation functions of these ions indicate that about one, two or three, three, and one hydrogen bond of each NH 4(1-1), NH4(1-2), NH4(2) and NH4(3) type ion is kept for 1.4, 21, 75, and 1.4 ps, respectively, before breakup of the hydrogen bond occurs a nd significant exchange of O atom hydrogen-bonded to the ion.