How Does Benzene in NaY Zeolite Couple to the Framework Vibrations?
Fabien Jousse,*,, Daniel P. Vercauteren, and Scott M. Auerbach,§
Computational Chemical Physics Group, Institute for Studies in Interface Science, Faculte´s UniVersitaires
Notre-Dame de la Paix, Rue de Bruxelles 61, B-5000 Namur, Belgium, and Departments of Chemistry and
Chemical Engineering, UniVersity of Massachusetts, Amherst, Massachusetts 01002
ReceiVed: December 7, 1999
Constrained energy minimization, equilibrium and nonequilibrium molecular dynamics calculations, and
constrained Monte Carlo simulations were used to determine the influence of the coupling between benzene
adsorbed in a NaY zeolite model (Si/Al ) 2) and the framework vibrations on benzene site-to-site dynamics.
Benzene at an SII site is strongly coupled to the nearby Na(II) cation, resulting in a decrease of the external
vibrational frequency of the center-of-mass of benzene away from this cation by 60 cm-1. Despite this coupling,
framework vibrations have remarkably little influence on the site-to-site rate constants of benzene. Although
with a fixed framework no dissipation of the excess kinetic energy of the adsorbed molecule can take place
and thus no thermalization to equilibrium, energy redistribution from the translational motion of benzene into
the internal degrees of freedom of the flexible molecule is seen to provide a good thermalization.
Intracrystalline self-diffusion of guest molecules in zeolites1,2
can be accurately reproduced by kinetic Monte Carlo3-5 or jump
diffusion6,7 models, as well as lattice gas8-10 or Ising models.11